feat(state): clean up per-bottle state on session end (except capability-block)

Previously every bottle launch left ~/.claude-bottle/state/<identity>/
behind forever — metadata.json on every run, plus per-bottle
Dockerfile + transcript snapshot on capability-block rebuilds. The
metadata accumulated debris across launches; the only state worth
keeping was the capability-block rebuild bundle.

Make cleanup the default; preserve only on capability-block.

- bottle_state.py: .preserve marker helpers (mark_preserved,
  is_preserved, clear_preserve_marker, preserve_marker_path) +
  cleanup_state(identity) that rm -rf's the per-bottle dir.
- capability_apply.apply_capability_change writes mark_preserved
  before teardown so cli.py's session-end cleanup keeps the dir.
- prepare.py clears any leftover marker at launch (start or resume),
  so a marker from a prior capability-block doesn't keep state
  alive past a subsequent normal session-end.
- cli/start.py runs the cleanup decision AFTER the launch context
  closes: if is_preserved → print resume hint; else cleanup_state.
  The resume hint moves out of the launch with-block (was previously
  printed unconditionally — would have misled the operator about
  whether state was actually kept).

Future-proof: cli.py never persists state speculatively. If the
agent wants to be resumable, it has to go through capability-block.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

claude_bottle/backend/docker/prepare.py

@@ -30,6 +30,7 @@ from .git_gate import DockerGitGate, git_gate_container_name
 from .bottle_state import (
     BottleMetadata,
     bottle_identity,
+    clear_preserve_marker,
     per_bottle_dockerfile,
     per_bottle_dockerfile_path,
     per_bottle_image_tag,
@@ -73,6 +74,10 @@ def resolve_plan(
         copy_cwd=spec.copy_cwd,
         started_at=datetime.now(timezone.utc).isoformat(),
     ))
+    # Clear any leftover preserve marker from a prior capability-block
+    # so this fresh launch can be cleaned up at session-end unless
+    # the agent triggers another capability-block.
+    clear_preserve_marker(slug)
 
     # PRD 0016 capability-block: if a per-bottle Dockerfile has been
     # written (via apply_capability_change), the base image becomes