feat(dashboard): Tab toggle + per-pane selection state

PRD 0019 chunk 3. The TUI now has two focusable panes — proposals
and agents — and `Tab` toggles which one the `j/k`/arrow keys
move through.

Each pane keeps its own selection index. Switching panes doesn't
lose the position in the other; the cursor (`>` + reverse-video
row) appears only in the focused pane. The label line on each
pane shows "(focused)" when active.

Footer reshuffled: `[Tab] switch pane  [j/k] move  [Enter] view
[a/m/r] proposal  [e/p] edit  [q] quit`. When the agents pane is
focused and there's no status message to display, the idle
status line surfaces the currently-selected agent (or "[no
active agents]" / "[no agent selected]" fallbacks) so the
operator knows what an agent-scoped edit verb will target after
chunk 4 wires them up.

Proposal action keys (a/m/r/Enter) are gated on the proposals
pane being focused — pressing them with the agents pane focused
is a no-op. e/p still use the global discover-and-prompt flow
for one more chunk; chunk 4 swaps them to read the agents-pane
selection.

tests/unit/test_dashboard_active_agents.py

@@ -224,5 +224,37 @@ class TestFormatAgentRow(unittest.TestCase):
         self.assertTrue(s.endswith("…"))
 
 
+class TestSelectionStatus(unittest.TestCase):
+    """Idle-state status-line text for the agents-pane focus
+    (PRD 0019 chunk 3). Empty when the proposals pane is focused;
+    surfaces the selected agent (or a clear placeholder) when the
+    agents pane is focused."""
+
+    def _agent(self, slug: str) -> dashboard.ActiveAgent:
+        return dashboard.ActiveAgent(
+            slug=slug, agent_name="x", started_at="", services=(),
+        )
+
+    def test_empty_when_proposals_focused(self):
+        s = dashboard._selection_status(
+            dashboard.PANE_PROPOSALS, [self._agent("a-1")], 0,
+        )
+        self.assertEqual("", s)
+
+    def test_no_agents_message_when_agents_pane_empty(self):
+        s = dashboard._selection_status(dashboard.PANE_AGENTS, [], 0)
+        self.assertEqual("[no active agents]", s)
+
+    def test_shows_selected_slug(self):
+        agents = [self._agent("a-1"), self._agent("b-2"), self._agent("c-3")]
+        s = dashboard._selection_status(dashboard.PANE_AGENTS, agents, 1)
+        self.assertEqual("[selected: b-2]", s)
+
+    def test_out_of_bounds_falls_back_to_no_selection(self):
+        agents = [self._agent("only")]
+        s = dashboard._selection_status(dashboard.PANE_AGENTS, agents, 99)
+        self.assertEqual("[no agent selected]", s)
+
+
 if __name__ == "__main__":
     unittest.main()