feat(dashboard): x stops a dashboard-owned bottle (PRD 0020 chunk 4)

Final PRD 0020 chunk. `x` on a focused agents-pane row tears
down the selected bottle if the dashboard owns it (started via
the chunk-2 `n` flow): pops `(cm, bottle, identity)` from the
main loop's bottles map, snapshots the transcript best-effort,
calls `cm.__exit__(None, None, None)` to drive the existing
compose-down + network-remove sequence, then `settle_state` to
honor any pre-existing preserve marker.

On a non-owned slug (discovered via `list_active_slugs` but not
in the dashboard's bottles dict — i.e., previous-dashboard or
external `./cli.py start` bottle), `x` is a no-op with a status
hint pointing at `./cli.py cleanup`. Matches the PRD's
cross-dashboard re-attach model: the dashboard can re-attach
either kind, but can only tear down its own.

The PRD's chunk 5 ("quit-cleanup") is satisfied by the existing
no-op behavior of `q` — per the user's resolved-question
answer, quit leaves bottles running unchanged. No code change
needed for that.

Footer surfaces `[x] stop`. 465 unit tests pass (1 new for the
non-owned no-op path; the owned path is integration territory
because it drives a real compose-down).

tests/unit/test_dashboard_active_agents.py

@@ -379,6 +379,24 @@ class TestBottleForSlug(unittest.TestCase):
         self.assertEqual("", hint)
 
 
+class TestStopBottleFlow(unittest.TestCase):
+    """Explicit per-bottle stop (PRD 0020 chunk 4). The non-owned
+    path is the one safe to test without curses + docker — the
+    owned path drives `cm.__exit__` against a real launch context
+    and belongs in integration tests."""
+
+    def test_non_owned_returns_cleanup_hint(self):
+        # stdscr is None here on purpose — the non-owned branch
+        # returns before any curses call.
+        msg = dashboard._stop_bottle_flow(
+            stdscr=None,  # type: ignore[arg-type]
+            bottles={},
+            slug="ghost-zzz",
+        )
+        self.assertIn("not dashboard-owned", msg)
+        self.assertIn("./cli.py cleanup", msg)
+
+
 class TestOperatorEditFlowGuards(_FakeHomeMixin, unittest.TestCase):
     """Chunk-4 contract: the edit flow refuses when the selected
     agent doesn't have the required sidecar running. The discover-