feat(cli): add commit command to snapshot running bottle state

Adds `./cli.py commit [<slug>]` which runs `docker commit` on the
active agent container and stores the resulting image tag in per-bottle
state. The next `./cli.py resume <slug>` automatically boots from the
committed snapshot instead of rebuilding from the Dockerfile, preserving
all in-container state across restarts and migrations.

- bottle_state: add write_committed_image / read_committed_image helpers
- docker/util: add commit_container wrapper around `docker commit`
- docker/launch: check for a committed image before the Dockerfile build
  step; fall back to normal build if the image is absent from the daemon
- cli/commit: new command with interactive slug picker; errors clearly on
  non-Docker backends
- 50 new unit tests covering all paths

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docs/prds/prd-new-commit-bottle-state.md

@@ -0,0 +1,136 @@
+# PRD prd-new: Commit bottle state to an image
+
+- **Status:** Draft
+- **Author:** Claude
+- **Created:** 2026-06-20
+- **Issue:** #194
+
+## Summary
+
+Add a `commit` CLI command that freezes a running Docker bottle's
+container state to a named Docker image. Operators can then resume the
+bottle from that exact filesystem snapshot, or export the image with
+`docker save` to migrate work to a different host.
+
+## Problem
+
+When a long-running agent session is interrupted — by a host reboot, a
+network failure, or a planned infrastructure migration — the in-progress
+container state is lost. `cli.py resume` rebuilds the agent image from
+the Dockerfile and reprovi-sions the bottle, but that returns the guest
+to its initial state, not to wherever the agent was mid-task.
+
+There is no mechanism today to capture "what's installed / configured
+inside the running container right now" and make it reproducible. The
+`capability-block` flow writes a new Dockerfile and marks the bottle for
+resume, but that only applies when the agent itself has requested a
+capability change; it doesn't help the operator who wants to take a
+snapshot before a planned host reboot or hardware migration.
+
+## Goals / Success Criteria
+
+- `./cli.py commit [<slug>]` takes a snapshot of the running Docker
+  agent container and stores it as a local Docker image.
+- Without a slug argument the command shows the same interactive picker
+  as `start` (the list of active slugs).
+- The committed image tag is stored in per-bottle state so that the next
+  `./cli.py resume <slug>` automatically uses the committed image instead
+  of rebuilding from the Dockerfile.
+- `mark_preserved` is called so the state dir survives the normal
+  session-end cleanup.
+- A `docker save` hint is printed so operators know how to export the
+  image for migration.
+- The command errors clearly on non-Docker backends (smolmachines does
+  not expose a container-level commit API in its current CLI surface).
+
+## Non-goals
+
+- Smolmachines or macOS-container backend support.
+- Automatic commit on agent exit.
+- Image push to a remote registry.
+- Storing the image tag in the manifest or sharing it between operators.
+
+## Design
+
+### Image tag
+
+`bot-bottle-committed-<slug>:latest` — namespaced under `bot-bottle-`
+to match existing image naming conventions; `committed` distinguishes it
+from the build-time image (`bot-bottle-claude:latest`) and the
+capability-block rebuild image (`bot-bottle-rebuilt-<identity>:latest`).
+
+### State storage
+
+A new plain-text file `committed-image` is added to the per-bottle state
+directory:
+
+```
+~/.bot-bottle/state/<identity>/
+    metadata.json
+    Dockerfile            (capability-block override; optional)
+    committed-image       (committed image tag; optional)
+    transcript/
+```
+
+`bottle_state.committed_image_path(identity)` returns the path.
+`write_committed_image` / `read_committed_image` are the read/write
+helpers, matching the existing `per_bottle_dockerfile` pattern.
+
+### `commit` command
+
+```
+./cli.py commit [<slug>]
+```
+
+1. Resolve slug (arg or interactive picker from `enumerate_active_agents`).
+2. Check metadata: if `backend` is set and is not `docker`, die with a
+   clear "not supported" error.
+3. Derive container name: `bot-bottle-<slug>` (matches the agent
+   provision plan's `instance_name` convention).
+4. Run `docker commit <container> bot-bottle-committed-<slug>:latest`.
+5. Write the image tag to `~/.bot-bottle/state/<slug>/committed-image`.
+6. Call `mark_preserved(<slug>)` so the state dir survives session-end.
+7. Print the resume hint and a `docker save` export example.
+
+### Resume from committed image
+
+`bot_bottle/backend/docker/launch.py` already rebuilds the agent image
+at the top of the `launch` context manager. The change is a check
+immediately before that step:
+
+```python
+committed = read_committed_image(plan.slug)
+if committed and docker_mod.image_exists(committed):
+    info(f"using committed image {committed!r}")
+    plan = dataclasses.replace(
+        plan,
+        agent_provision=dataclasses.replace(plan.agent_provision, image=committed),
+    )
+else:
+    docker_mod.build_image(plan.image, _REPO_DIR, dockerfile=plan.dockerfile_path)
+```
+
+Replacing `agent_provision.image` propagates to `plan.image` (a
+property) and from there to the Compose spec renderer's `_agent_service`
+→ `image:` field, so the container boots from the committed snapshot.
+The build step is skipped entirely when a committed image is found and
+exists locally.
+
+If the committed image has been deleted from the local daemon (e.g.
+after `docker rmi` or a `docker system prune`), the launch falls back
+to a normal Dockerfile build, matching the pre-commit behavior.
+
+## Testing strategy
+
+- Unit tests for `write_committed_image` / `read_committed_image` in
+  `tests/unit/test_bottle_state.py`, using the existing `_FakeHomeMixin`
+  pattern.
+- Unit tests for `commit_container` in `tests/unit/test_docker_util_image.py`,
+  mocking `subprocess.run` and asserting on the `docker commit` argv.
+- Unit tests for `cmd_commit` argument parsing and the "unsupported
+  backend" error path, mocking `enumerate_active_agents` and
+  `commit_container`.
+- Unit tests for the launch-step committed-image branch: patch
+  `read_committed_image` to return a tag, patch `image_exists` to return
+  True, and assert that `build_image` is not called and `plan.image` is
+  overridden.