refactor(backend): move per-provider provisioning onto AgentProvider

BottleBackend.provision now resolves the provider plugin from the
plan and dispatches prompt / skills / declarative-apply /
supervise-mcp through it. The four hooks the docker + smolmachines
backends used to override (provision_skills, provision_prompt,
provision_provider_auth, provision_supervise) are gone — the
duplicated 50-line implementations under
backend/{docker,smolmachines}/provision/{skills,prompt,
provider_auth,supervise}.py are deleted.

Each backend gains a small supervise_mcp_url(plan) override so the
provider plugin can run `claude mcp add` / `codex mcp add`
against the right URL: docker returns
http://{SUPERVISE_HOSTNAME}:{SUPERVISE_PORT}/ on the compose
network alias; smolmachines returns plan.agent_supervise_url which
launch.py already pins to a host-loopback port.

Removes tests/unit/test_provision_supervise.py — the URL it
asserted on now lives on the backend, with no equivalent
standalone surface to test against (it's covered by the broader
plan / launch integration tests).

bot_bottle/backend/docker/provision/__init__.py

@@ -1,8 +1,11 @@
-"""Per-provisioner modules for the Docker backend.
+"""Backend-infrastructure provisioners for the Docker backend.
 
-Each module exports one top-level function:
-    provision_<thing>(plan: DockerBottlePlan, bottle: Bottle) -> ...
+Per PRD 0050 the per-provider provisioning steps (prompt, skills,
+declarative provision-plan apply, supervise MCP registration) live on
+the `AgentProvider` plugin under `bot_bottle/contrib/`. The modules
+left in this subpackage handle only the steps that are
+backend-specific:
 
-`DockerBottleBackend.provision_*` methods delegate to these. The
-abstract `BottleBackend.provision_*` surface is unchanged; this
-subpackage exists only to keep `backend.py` from being a god-file."""
+  - ca.py   — install per-bottle CA bundle into the guest trust store
+  - git.py  — copy host cwd `.git` into the guest when --cwd is used
+"""