feat(cli): add launch selector TUI for start command (PRD 0051)

- Add bot_bottle/cli/tui.py: curses filter-select picker that opens
  /dev/tty directly so it works with redirected stdout/stdin
- Make `name` positional optional (nargs="?") in cmd_start; show agent
  picker when absent
- Show backend picker when no --backend flag and BOT_BOTTLE_BACKEND is
  unset; skip when either is explicit or the env var is present
- Add tests/unit/test_cli_tui.py covering _filter_items logic and
  short-circuit paths (empty list, unavailable tty)
- Add tests/unit/test_cli_start_selector.py covering all four dispatch
  combinations (both explicit, agent-absent, backend-absent, both-absent)
  and cancel semantics
- Activate PRD 0051

tests/unit/test_cli_start_selector.py

@@ -0,0 +1,143 @@
+"""Unit: cmd_start selector dispatch (PRD 0051).
+
+Tests that cmd_start calls filter_select when name / backend are absent,
+skips them when both are explicit, and returns 0 on cancel.
+
+All actual launch work is stubbed so no container is created.
+"""
+
+from __future__ import annotations
+
+import os
+import sys
+import types
+import unittest
+from unittest.mock import MagicMock, patch, call
+
+import bot_bottle.cli.start as start_mod
+import bot_bottle.cli.tui as tui_mod
+
+
+def _make_manifest(agent_names: list[str]):
+    manifest = MagicMock()
+    manifest.agents = {name: MagicMock() for name in agent_names}
+    return manifest
+
+
+class TestCmdStartSelector(unittest.TestCase):
+    """Drive cmd_start with a minimal set of stubs."""
+
+    def setUp(self):
+        # Stub Manifest.resolve so no on-disk manifest is needed.
+        self._manifest = _make_manifest(["researcher", "implementer"])
+        self._resolve_patch = patch(
+            "bot_bottle.cli.start.Manifest.resolve",
+            return_value=self._manifest,
+        )
+        self._resolve_patch.start()
+
+        # Stub _launch_bottle so no real container work happens.
+        self._launch_patch = patch(
+            "bot_bottle.cli.start._launch_bottle",
+            return_value=0,
+        )
+        self._launch_mock = self._launch_patch.start()
+
+        # Stub filter_select to avoid opening /dev/tty.
+        self._tui_patch = patch.object(tui_mod, "filter_select")
+        self._tui_mock = self._tui_patch.start()
+
+        # Ensure BOT_BOTTLE_BACKEND is absent so the backend picker fires.
+        self._env_patch = patch.dict(os.environ, {}, clear=False)
+        self._env_patch.start()
+        os.environ.pop("BOT_BOTTLE_BACKEND", None)
+
+    def tearDown(self):
+        self._resolve_patch.stop()
+        self._launch_patch.stop()
+        self._tui_patch.stop()
+        self._env_patch.stop()
+
+    # ------------------------------------------------------------------
+    # Both explicit — no picker shown
+    # ------------------------------------------------------------------
+
+    def test_both_explicit_skips_picker(self):
+        self._tui_mock.return_value = "researcher"
+        rc = start_mod.cmd_start(["--backend=docker", "researcher"])
+        self.assertEqual(0, rc)
+        self._tui_mock.assert_not_called()
+        self._launch_mock.assert_called_once()
+        _, kwargs = self._launch_mock.call_args
+        self.assertEqual("docker", kwargs["backend_name"])
+
+    # ------------------------------------------------------------------
+    # Agent absent → agent picker fires; backend explicit
+    # ------------------------------------------------------------------
+
+    def test_agent_absent_shows_agent_picker(self):
+        self._tui_mock.return_value = "researcher"
+        rc = start_mod.cmd_start(["--backend=docker"])
+        self.assertEqual(0, rc)
+        self._tui_mock.assert_called_once()
+        call_kwargs = self._tui_mock.call_args
+        self.assertEqual(["implementer", "researcher"], call_kwargs[0][0])
+        self.assertIn("agent", call_kwargs[1]["title"].lower())
+
+    def test_agent_picker_cancel_returns_0(self):
+        self._tui_mock.return_value = None
+        rc = start_mod.cmd_start(["--backend=docker"])
+        self.assertEqual(0, rc)
+        self._launch_mock.assert_not_called()
+
+    # ------------------------------------------------------------------
+    # Agent explicit, backend absent → backend picker fires
+    # ------------------------------------------------------------------
+
+    def test_backend_absent_shows_backend_picker(self):
+        self._tui_mock.return_value = "docker"
+        rc = start_mod.cmd_start(["researcher"])
+        self.assertEqual(0, rc)
+        self._tui_mock.assert_called_once()
+        call_kwargs = self._tui_mock.call_args
+        self.assertIn("backend", call_kwargs[1]["title"].lower())
+
+    def test_backend_picker_cancel_returns_0(self):
+        self._tui_mock.return_value = None
+        rc = start_mod.cmd_start(["researcher"])
+        self.assertEqual(0, rc)
+        self._launch_mock.assert_not_called()
+
+    def test_bot_bottle_backend_env_skips_backend_picker(self):
+        os.environ["BOT_BOTTLE_BACKEND"] = "docker"
+        try:
+            rc = start_mod.cmd_start(["researcher"])
+        finally:
+            os.environ.pop("BOT_BOTTLE_BACKEND", None)
+        self.assertEqual(0, rc)
+        self._tui_mock.assert_not_called()
+
+    # ------------------------------------------------------------------
+    # Both absent → agent picker then backend picker
+    # ------------------------------------------------------------------
+
+    def test_both_absent_shows_both_pickers_in_order(self):
+        self._tui_mock.side_effect = ["researcher", "docker"]
+        rc = start_mod.cmd_start([])
+        self.assertEqual(0, rc)
+        self.assertEqual(2, self._tui_mock.call_count)
+        first_title = self._tui_mock.call_args_list[0][1]["title"].lower()
+        second_title = self._tui_mock.call_args_list[1][1]["title"].lower()
+        self.assertIn("agent", first_title)
+        self.assertIn("backend", second_title)
+
+    def test_both_absent_agent_cancel_skips_backend_picker(self):
+        self._tui_mock.side_effect = [None]
+        rc = start_mod.cmd_start([])
+        self.assertEqual(0, rc)
+        self.assertEqual(1, self._tui_mock.call_count)
+        self._launch_mock.assert_not_called()
+
+
+if __name__ == "__main__":
+    unittest.main()