fix: label becomes the full slug; re-prompt on collision

When a label is given it is now used verbatim as the slug (no random
suffix), so two launches with the same label collide by design.  The
CLI re-prompts via the TUI name modal with a disclaimer when the
candidate slug is already in use among running bottles.

tests/unit/test_cli_start_selector.py

@@ -14,6 +14,7 @@ from unittest.mock import MagicMock, patch
 
 import bot_bottle.cli.start as start_mod
 import bot_bottle.cli.tui as tui_mod
+from bot_bottle.backend import ActiveAgent
 
 
 def _make_manifest(agent_names: list[str]):
@@ -133,5 +134,63 @@ class TestCmdStartSelector(unittest.TestCase):
         self._launch_mock.assert_not_called()
 
 
+def _active_agent(slug: str) -> ActiveAgent:
+    return ActiveAgent(
+        backend_name="docker",
+        slug=slug,
+        agent_name="demo",
+        started_at="2026-01-01T00:00:00+00:00",
+        services=(),
+    )
+
+
+class TestCmdStartLabelCollision(unittest.TestCase):
+    """cmd_start re-prompts when the label's slug is already running."""
+
+    def setUp(self):
+        self._manifest = _make_manifest(["researcher"])
+        patch("bot_bottle.cli.start.Manifest.resolve", return_value=self._manifest).start()
+        self._launch_mock = patch(
+            "bot_bottle.cli.start._launch_bottle", return_value=0,
+        ).start()
+        self.addCleanup(patch.stopall)
+
+    def test_no_collision_proceeds_without_reprompt(self):
+        with (
+            patch.object(tui_mod, "name_color_modal", return_value=("researcher", "")) as modal,
+            patch("bot_bottle.cli.start.enumerate_active_agents", return_value=[]),
+        ):
+            rc = start_mod.cmd_start(["researcher"])
+        self.assertEqual(0, rc)
+        modal.assert_called_once()
+        self._launch_mock.assert_called_once()
+
+    def test_collision_reprompts_with_disclaimer(self):
+        collision_agent = _active_agent("researcher")
+        call_count = 0
+
+        def _modal(default_label, *, disclaimer="", **_kw):
+            nonlocal call_count
+            call_count += 1
+            if call_count == 1:
+                return "researcher", ""
+            return "researcher-2", ""
+
+        with (
+            patch.object(tui_mod, "name_color_modal", side_effect=_modal) as modal,
+            patch(
+                "bot_bottle.cli.start.enumerate_active_agents",
+                side_effect=[[collision_agent], []],
+            ),
+        ):
+            rc = start_mod.cmd_start(["researcher"])
+
+        self.assertEqual(0, rc)
+        self.assertEqual(2, modal.call_count)
+        second_call_kwargs = modal.call_args_list[1][1]
+        self.assertIn("researcher", second_call_kwargs.get("disclaimer", ""))
+        self.assertIn("already in use", second_call_kwargs.get("disclaimer", ""))
+
+
 if __name__ == "__main__":
     unittest.main()