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/capability_apply.py

@@ -37,6 +37,7 @@ from pathlib import Path
 
 from ...log import info, warn
 from .bottle_state import (
+    mark_preserved,
     per_bottle_dockerfile,
     per_bottle_dockerfile_path,
     transcript_snapshot_dir,
@@ -117,6 +118,11 @@ def apply_capability_change(slug: str, new_dockerfile: str) -> tuple[str, str]:
     _snapshot_transcript(slug)
     _push_working_tree(slug)
     write_per_bottle_dockerfile(slug, new_dockerfile)
+    # Set the preserve marker BEFORE teardown so cli.py's session-end
+    # cleanup sees it and keeps the state dir intact for the
+    # operator's `cli.py resume <identity>`. Without the marker the
+    # state dir would be deleted as part of normal session end.
+    mark_preserved(slug)
     _teardown_bottle(slug)
 
     return before, new_dockerfile