feat(dashboard): auto-focus next agent on stop, or close pane

After `x` stops a dashboard-owned bottle, slide focus to the
next agent in the agents pane (the one filling the stopped
row, or the new last row if the stopped was last) and respawn
the right pane with that agent's claude session via `--continue`.
If no agents remain, close the right pane via `tmux kill-pane`.

Two new helpers:

  - `_tmux_close_right_pane(tmux_state)` — kills the tracked
    pane (if it exists) and clears pane_id / slug.
  - `_pick_next_after_stop(agents_before, selected_index,
    stopped_slug)` — pure chooser returning (new_index, agent)
    or None. Tested directly.

Outside tmux, only the selected_agent index slides; no
auto-attach (foreground handoff would take over the terminal,
disruptive). 485 unit tests pass (6 new for the pick helper).

tests/unit/test_dashboard_active_agents.py

@@ -379,6 +379,55 @@ class TestBottleForSlug(unittest.TestCase):
         self.assertEqual("", hint)
 
 
+class TestPickNextAfterStop(unittest.TestCase):
+    """After `x` stops a bottle, the dashboard slides focus to
+    the next agent — the one filling the stopped row, or the
+    new last row if the stopped was last. Pure helper, easy
+    to unit-test."""
+
+    def _agent(self, slug: str) -> dashboard.ActiveAgent:
+        return dashboard.ActiveAgent(
+            slug=slug, agent_name=slug, started_at="", services=(),
+        )
+
+    def test_empty_list_returns_none(self):
+        self.assertIsNone(
+            dashboard._pick_next_after_stop([], 0, "anything"),
+        )
+
+    def test_only_agent_being_stopped_returns_none(self):
+        # Stopping the last agent → nothing to focus.
+        agents = [self._agent("only")]
+        self.assertIsNone(
+            dashboard._pick_next_after_stop(agents, 0, "only"),
+        )
+
+    def test_middle_row_slides_up_to_same_index(self):
+        agents = [self._agent("a"), self._agent("b"), self._agent("c")]
+        # Cursor was on "b" at index 1; stopping "b" → "c" now sits
+        # at index 1 and takes focus.
+        out = dashboard._pick_next_after_stop(agents, 1, "b")
+        self.assertEqual((1, self._agent("c")), out)
+
+    def test_last_row_wraps_to_new_last(self):
+        agents = [self._agent("a"), self._agent("b"), self._agent("c")]
+        # Cursor on "c" at index 2; stopping "c" leaves a 2-agent
+        # list — index 2 is out of bounds, fall back to new last (1).
+        out = dashboard._pick_next_after_stop(agents, 2, "c")
+        self.assertEqual((1, self._agent("b")), out)
+
+    def test_first_row(self):
+        agents = [self._agent("a"), self._agent("b")]
+        out = dashboard._pick_next_after_stop(agents, 0, "a")
+        self.assertEqual((0, self._agent("b")), out)
+
+    def test_clamps_negative_selection(self):
+        # Defensive: a stale negative index doesn't crash.
+        agents = [self._agent("a"), self._agent("b")]
+        out = dashboard._pick_next_after_stop(agents, -1, "a")
+        self.assertEqual((0, self._agent("b")), out)
+
+
 class TestTmuxPaneArgvBuilders(unittest.TestCase):
     """Pure argv builders for the tmux split-pane integration
     (PRD 0021 chunk 2). The subprocess invocation itself is