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>

tests/unit/test_capability_apply.py

@@ -107,6 +107,14 @@ class TestApplyCapabilityChange(_FakeHomeMixin, unittest.TestCase):
             self._calls,
         )
 
+    def test_marks_preserved_before_teardown(self):
+        # cli.py's session-end cleanup reads the marker after the
+        # bottle is torn down. The marker must therefore be written
+        # before teardown — otherwise the cleanup would see no
+        # marker and rm the state dir we just populated.
+        apply_capability_change("dev", "FROM new\n")
+        self.assertTrue(bottle_state.is_preserved("dev"))
+
     def test_first_change_falls_back_to_repo_dockerfile_for_before(self):
         # No per-bottle override yet — before-diff comes from the
         # repo's Dockerfile.