feat(state): preserve on crash + always snapshot transcript

Extends the preserve-on-capability-block design to also preserve
state on agent crash, and snapshots the transcript on every
teardown so any resume (crash or capability-block) gets a warm
claude session — not a cold start.

- capability_apply: rename _snapshot_transcript → snapshot_transcript
  (public; reused below). No behavior change in the capability path.
- cli/start.py: capture bottle.exec_claude's exit code; while the
  container is still alive (inside the launch context):
    * always snapshot_transcript(identity)
    * if exit_code != 0, mark_preserved(identity)
  Then the existing _settle_state runs after teardown.

Now the preservation matrix is:

  exit 0   (clean)          → snapshot + cleanup state
  exit ≠0  (crash, Ctrl-C)  → snapshot + preserve + show resume hint
  capability-block          → (already snapshotted/preserved by apply
                               before teardown; this path is a no-op
                               because the container is already gone
                               by the time exec_claude returns)

snapshot_transcript is best-effort — capability-block's earlier
snapshot is not clobbered when the container is already torn down,
and a missing /home/node/.claude is a warn + skip.

Tested behavior: clean exit doesn't preserve, non-zero exit
(including SIGINT/130 and SIGKILL/137) preserves; empty identity
no-ops both helpers.

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

tests/unit/test_capability_apply.py

@@ -63,7 +63,7 @@ class TestApplyCapabilityChange(_FakeHomeMixin, unittest.TestCase):
         # job is to sequence write + snapshot + push + teardown; we
         # validate that sequence here, not the docker primitives.
         self._calls: list[str] = []
-        self._orig_snapshot = capability_apply._snapshot_transcript
+        self._orig_snapshot = capability_apply.snapshot_transcript
         self._orig_push = capability_apply._push_working_tree
         self._orig_teardown = capability_apply._teardown_bottle
 
@@ -76,12 +76,12 @@ class TestApplyCapabilityChange(_FakeHomeMixin, unittest.TestCase):
         def stub_teardown(slug):
             self._calls.append(f"teardown:{slug}")
 
-        capability_apply._snapshot_transcript = stub_snapshot  # type: ignore[assignment]
+        capability_apply.snapshot_transcript = stub_snapshot  # type: ignore[assignment]
         capability_apply._push_working_tree = stub_push  # type: ignore[assignment]
         capability_apply._teardown_bottle = stub_teardown  # type: ignore[assignment]
 
     def tearDown(self):
-        capability_apply._snapshot_transcript = self._orig_snapshot  # type: ignore[assignment]
+        capability_apply.snapshot_transcript = self._orig_snapshot  # type: ignore[assignment]
         capability_apply._push_working_tree = self._orig_push  # type: ignore[assignment]
         capability_apply._teardown_bottle = self._orig_teardown  # type: ignore[assignment]
         self._teardown_fake_home()