feat(dashboard): stop hook clears tmux state + right-pane row marker

PRD 0021 chunk 4 (final). Two adjustments to close the
split-pane loop:

1. `_stop_bottle_flow` clears `tmux_state['slug']` when the
   stopped bottle was the right-pane occupant. The pane itself
   stays in place (claude exits with "container not found");
   the operator presses Enter on a different agent to
   repurpose it via respawn-pane.
2. `_render` accepts `right_pane_slug` and marks the matching
   agents-pane row with a `*` prefix + A_BOLD (when it's not
   also the focused row — focused selection still wins for
   visibility). Gives the operator a clear visual link
   between which agent the dashboard says is "active right
   now" and which one is visible to their right.

Wired through `_main_loop`: passes `tmux_state` to
`_stop_bottle_flow` on `x`, and `tmux_state.get('slug')` to
`_render` on every tick.

479 unit tests pass (1 new for the tmux_state-preservation
on non-owned stop). PRD 0021 implementation complete pending
merge.

tests/unit/test_dashboard_active_agents.py

@@ -455,6 +455,20 @@ class TestStopBottleFlow(unittest.TestCase):
         self.assertIn("not dashboard-owned", msg)
         self.assertIn("./cli.py cleanup", msg)
 
+    def test_non_owned_does_not_touch_tmux_state(self):
+        # PRD 0021: a stop on an unknown slug should never clear
+        # the right-pane occupant tracking, even if the slugs
+        # happen to match (defensive — non-owned can't be in the
+        # right pane via the dashboard's normal flow anyway).
+        tmux_state = {"pane_id": "%5", "slug": "live-bbb"}
+        dashboard._stop_bottle_flow(
+            stdscr=None,  # type: ignore[arg-type]
+            bottles={},
+            slug="ghost-zzz",
+            tmux_state=tmux_state,
+        )
+        self.assertEqual({"pane_id": "%5", "slug": "live-bbb"}, tmux_state)
+
 
 class TestOperatorEditFlowGuards(_FakeHomeMixin, unittest.TestCase):
     """Chunk-4 contract: the edit flow refuses when the selected