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>

bot_bottle/bottle_state.py

@@ -43,6 +43,7 @@ from . import supervise as _supervise
 # Directory layout: ~/.bot-bottle/state/<identity>/...
 _STATE_SUBDIR = "state"
 _PER_BOTTLE_DOCKERFILE_NAME = "Dockerfile"
+_COMMITTED_IMAGE_NAME = "committed-image"
 _TRANSCRIPT_SUBDIR = "transcript"
 # Per-sidecar scratch subdirs. PRD 0018 chunk 2: bind-mount sources
 # live here so chunk 3's `docker compose up` can find them at stable
@@ -179,6 +180,32 @@ def write_per_bottle_dockerfile(identity: str, content: str) -> Path:
     return p
 
 
+def committed_image_path(identity: str) -> Path:
+    return bottle_state_dir(identity) / _COMMITTED_IMAGE_NAME
+
+
+def write_committed_image(identity: str, image_tag: str) -> Path:
+    """Persist the committed image tag for `identity`. The next
+    `cli.py resume <identity>` will boot from this image instead of
+    rebuilding from the Dockerfile."""
+    path = committed_image_path(identity)
+    path.parent.mkdir(parents=True, exist_ok=True)
+    path.write_text(image_tag.strip() + "\n")
+    path.chmod(0o644)
+    return path
+
+
+def read_committed_image(identity: str) -> str | None:
+    """Return the committed image tag for `identity`, or None if no
+    commit has been recorded. Used by the Docker launch step to skip
+    the Dockerfile build when a committed snapshot exists."""
+    path = committed_image_path(identity)
+    if not path.is_file():
+        return None
+    tag = path.read_text().strip()
+    return tag or None
+
+
 def per_bottle_image_tag(identity: str) -> str:
     """Image tag for a rebuilt bottle. Distinct from the base
     bot-bottle-claude:latest so per-bottle rebuilds don't collide in
@@ -314,6 +341,7 @@ __all__ = [
     "bottle_state_dir",
     "cleanup_state",
     "clear_preserve_marker",
+    "committed_image_path",
     "egress_state_dir",
     "git_gate_state_dir",
     "is_preserved",
@@ -323,9 +351,11 @@ __all__ = [
     "per_bottle_dockerfile_path",
     "per_bottle_image_tag",
     "preserve_marker_path",
+    "read_committed_image",
     "read_metadata",
     "supervise_state_dir",
     "transcript_snapshot_dir",
+    "write_committed_image",
     "write_metadata",
     "write_per_bottle_dockerfile",
 ]