docs(prd): expand user-provider-plugins to cover Dockerfile convention and provisioning methods

docs/prds/prd-new-user-provider-plugins.md

@@ -12,10 +12,16 @@ again at launch. Users who want to run a different agent (Gemini, Aider, a custo
 local model wrapper) cannot add a provider without forking the package.
 
 This PRD opens the registry to user-defined plugins. A plugin placed at
-`~/.bot-bottle/contrib/<name>/agent_provider.py` is discovered and loaded at launch
-time. The manifest accepts any non-empty template string that names a built-in or
-resolves to a user plugin at that path. No changes to the built-in providers or the
-internal `bot_bottle/contrib/` layout.
+`~/.bot-bottle/contrib/<name>/` is discovered and loaded at launch time. The manifest
+accepts any non-empty template string that names a built-in or resolves to a user
+plugin at that path.
+
+Alongside discovery, this PRD moves CA and git provisioning out of the Docker backend
+and into the `AgentProvider` ABC as overridable methods. The current standalone
+`provision/ca.py` and `provision/git.py` files in the Docker backend are deleted;
+their logic becomes the default implementations on the ABC. This lets exotic provider
+images (different base OS, different user, non-standard trust mechanism) override
+provisioning freely without the abstraction fighting them.
 
 The preceding commit on this PR moves `codex_auth.py` from `bot_bottle/` into
 `bot_bottle/contrib/codex/` — a clean-up that fits naturally here since this PR
@@ -32,33 +38,49 @@ be "cheap to add" — but "cheap" today still means a pull request against the b
 repo, not a drop-in file in the user's home directory. The filesystem layout is already
 the right shape; the discovery step is missing.
 
+Beyond discovery, the Docker backend's `provision_ca` and `provision_git` functions
+bake in Debian-specific commands (`update-ca-certificates`) and a hardcoded container
+user (`node`). A user plugin that runs as a different user, or on a different base OS,
+silently gets the wrong provisioning with no way to correct it short of forking.
+
 ## Goals / Success Criteria
 
 1. A user places `~/.bot-bottle/contrib/<name>/agent_provider.py` — a file that exports
    a class inheriting `AgentProvider` — sets `agent_provider.template: <name>` in a
    bottle's frontmatter, and launches a bottle using that provider with no changes to
    the bot-bottle source.
-2. The manifest validator accepts any non-empty template string. Unknown templates that
+2. The plugin directory may also contain a `Dockerfile` at
+   `~/.bot-bottle/contrib/<name>/Dockerfile`; the existing three-tier Dockerfile cascade
+   (per-bottle override → manifest `dockerfile:` field → provider default) uses this
+   path as the provider default for user plugins.
+3. The manifest validator accepts any non-empty template string. Unknown templates that
    resolve to no user plugin still raise a clear error, but at launch (via `get_provider`)
    rather than at manifest-load time.
-3. Built-in provider knobs (`auth_token` → claude only; `forward_host_credentials` →
+4. Built-in provider knobs (`auth_token` → claude only; `forward_host_credentials` →
    codex only) are guarded to built-in template names. Bottles using a user provider
    may set neither knob.
-4. `get_provider(template)` checks `~/.bot-bottle/contrib/<template>/agent_provider.py`
+5. `get_provider(template)` checks `~/.bot-bottle/contrib/<template>/agent_provider.py`
    before the built-ins, so a user can shadow a built-in for local testing.
-5. A clear `ValueError` is raised if the user plugin file exists but contains no
+6. A clear `ValueError` is raised if the user plugin file exists but contains no
    `AgentProvider` subclass.
+7. `AgentProvider` gains `provision_ca(self, bottle, plan)` and
+   `provision_git(self, bottle, plan)` with default implementations that reproduce
+   current Docker/Debian/node behavior. Built-in providers inherit the defaults
+   unchanged. User plugins override either method when their image diverges.
+8. `bot_bottle/backend/docker/provision/ca.py` and
+   `bot_bottle/backend/docker/provision/git.py` are deleted. The Docker backend base
+   class calls `provider.provision_ca(bottle, plan)` and
+   `provider.provision_git(bottle, plan)` directly.
 
 ## Non-goals
 
 - Packaging or distributing user plugins as installable Python packages.
 - A plugin registry, index, or discovery beyond the filesystem path convention.
 - Adding a third built-in provider.
-- Changing the `AgentProvider` ABC contract — user plugins implement the same abstract
-  methods as `ClaudeAgentProvider` and `CodexAgentProvider`.
 - Validating that user plugin images, Dockerfiles, or commands exist before launch
   (same policy as built-ins).
 - Sandboxing user plugin code — plugins run with full Python interpreter access.
+- Per-provider opt-out of the egress sidecar or network provisioning (follow-on).
 
 ## Scope
 
@@ -70,26 +92,45 @@ the right shape; the discovery step is missing.
 - `_load_user_plugin` uses `importlib.util.spec_from_file_location` to load the module
   and returns the first `AgentProvider` subclass found in its `__dict__`. Raises
   `ValueError` if the file exists but exports no subclass.
+- The Dockerfile cascade in the Docker backend's `resolve_plan()` uses
+  `~/.bot-bottle/contrib/<template>/Dockerfile` as the provider default for user
+  plugins (the same slot currently occupied by `Dockerfile.claude` / `Dockerfile.codex`
+  for built-ins).
 - `manifest_agent.AgentProvider.from_dict`: the `template not in PROVIDER_TEMPLATES`
   check is removed; the two built-in-specific knob guards (`auth_token` → claude,
   `forward_host_credentials` → codex) are tightened to `template in PROVIDER_TEMPLATES`
   so they are skipped for user-defined names.
 - `PROVIDER_TEMPLATES` remains in `agent_provider.py` as the set of built-in names for
   use by tests and any enumeration callers.
+- `AgentProvider` ABC gains:
+  ```python
+  def provision_ca(self, bottle: Bottle, plan: BottlePlan) -> None: ...
+  def provision_git(self, bottle: Bottle, plan: BottlePlan) -> None: ...
+  ```
+  Default implementations reproduce the current `provision/ca.py` and
+  `provision/git.py` logic exactly (Debian `update-ca-certificates`, `node` user,
+  `/home/node` home).
+- `bot_bottle/backend/docker/provision/ca.py` and
+  `bot_bottle/backend/docker/provision/git.py` deleted. The Docker backend base
+  class substitutes direct calls to the provider methods.
 - Unit tests for the discovery path:
   - Plugin found and loaded → correct `AgentProvider` instance returned.
   - Plugin file exists but exports no subclass → `ValueError`.
   - Unknown template with no user plugin → `ValueError` from `get_provider`.
   - Built-in template name still works normally even when no user plugin exists.
+- Unit tests for the provisioning delegation:
+  - A provider subclass that overrides `provision_ca` has its override called.
+  - A provider subclass that overrides `provision_git` has its override called.
 - One paragraph added to `README.md` under a new "Custom providers" section describing
-  the `~/.bot-bottle/contrib/<name>/agent_provider.py` convention and pointing at the
-  existing contrib providers as reference implementations.
+  the `~/.bot-bottle/contrib/<name>/` convention (both `agent_provider.py` and
+  `Dockerfile`), the `provision_ca` / `provision_git` override points, and pointing at
+  the existing contrib providers as reference implementations.
 
 ### Out of scope
 
 - Hot-reloading plugins during a running session.
 - Plugin versioning or dependency declaration.
-- Changes to smolmachines or Docker backend provisioning paths.
+- Changes to the smolmachines backend provisioning path.
 
 ## Proposed design
 
@@ -136,6 +177,49 @@ def _load_user_plugin(template: str) -> AgentProvider | None:
     )
 ```
 
+### Dockerfile convention for user plugins
+
+`resolve_plan()` in the Docker backend already has a three-tier cascade. For user
+plugins the provider-default slot is filled by:
+
+```python
+Path.home() / ".bot-bottle" / "contrib" / template / "Dockerfile"
+```
+
+Per-bottle overrides and manifest `dockerfile:` fields continue to take precedence.
+
+### Provisioning methods on `AgentProvider`
+
+```python
+class AgentProvider(ABC):
+    ...
+    def provision_ca(self, bottle: Bottle, plan: BottlePlan) -> None:
+        """Install the egress MITM CA into the agent container's trust store.
+        Override for non-Debian base images or non-standard trust mechanisms."""
+        cert_host_path, label = select_ca_cert(plan.egress_plan)
+        bottle.cp_in(str(cert_host_path), AGENT_CA_PATH)
+        bottle.exec(
+            f"chmod 644 {AGENT_CA_PATH} && update-ca-certificates",
+            user="root",
+        )
+        log_ca_fingerprint(cert_host_path, label)
+
+    def provision_git(self, bottle: Bottle, plan: BottlePlan) -> None:
+        """Configure git inside the agent container.
+        Override for images that run as a different user or use a non-standard home."""
+        _provision_cwd_git(plan, bottle)
+        _provision_git_gate_config(plan, bottle)
+        _provision_git_user(plan, bottle)
+```
+
+The Docker backend base class replaces the direct calls to the old standalone
+functions with:
+
+```python
+provider.provision_ca(bottle, plan)
+provider.provision_git(bottle, plan)
+```
+
 ### Manifest validation change
 
 In `manifest_agent.AgentProvider.from_dict`, remove the hard rejection:
@@ -172,10 +256,7 @@ if forward_host_credentials and template not in PROVIDER_TEMPLATES:
 
 ## Open questions
 
-1. **Shadow order.** This PRD puts user plugins before built-ins, allowing local
-   overrides. If the preference is built-ins-first (to prevent accidental shadowing),
-   swap the order and document accordingly.
-2. **`BOT_BOTTLE_CONTRIB_DIR` env var.** Omitted for now — `~/.bot-bottle/contrib/`
+1. **`BOT_BOTTLE_CONTRIB_DIR` env var.** Omitted for now — `~/.bot-bottle/contrib/`
    is consistent with the rest of the user config layout. Revisit if the need surfaces.
 
 ## References
@@ -184,3 +265,5 @@ if forward_host_credentials and template not in PROVIDER_TEMPLATES:
 - PRD 0048 — SSH deploy key provisioning (the `contrib/` convention)
 - `bot_bottle/agent_provider.py` — `get_provider`, `PROVIDER_TEMPLATES`, `AgentProvider` ABC
 - `bot_bottle/manifest_agent.py` — template validation that this PRD relaxes
+- `bot_bottle/backend/docker/provision/ca.py` — current CA provisioner (to be deleted)
+- `bot_bottle/backend/docker/provision/git.py` — current git provisioner (to be deleted)