feat(#269): separate agent and bottle selection at launch time

- `bottle:` in agent frontmatter is now optional; agents without it
  are portable and require bottles to be selected at launch.
- Adds `filter_multiselect` to `tui.py`: multi-select picker with
  ordered selection list, Space/Enter to toggle, Ctrl-D to confirm.
- `ManifestIndex` gains `all_bottle_names` and `load_for_agent` accepts
  `bottle_names: tuple[str, ...]` to merge bottles in order at runtime.
- `merge_bottles_runtime` in `manifest_extends.py` applies the same
  field-merge rules as `extends:` to pre-resolved bottle objects.
- `BottleSpec` gains `bottle_names`; `_validate` and `write_launch_metadata`
  thread it through so `resume` replays the same bottle configuration.
- `cmd_start` shows the bottle multiselect after agent selection,
  pre-populated from the agent's `bottle:` field when present.
- Existing agents with `bottle:` declared continue to work unchanged.

tests/unit/test_cli_tui.py

@@ -1,4 +1,4 @@
-"""Unit tests for bot_bottle.cli.tui — filter_select internals.
+"""Unit tests for bot_bottle.cli.tui — filter_select and filter_multiselect.
 
 We test the pure-Python logic (_filter_items, cursor movement, confirm,
 cancel) by exercising the internal helpers directly, without spinning up
@@ -9,7 +9,7 @@ from __future__ import annotations
 
 import unittest
 
-from bot_bottle.cli.tui import _filter_items, filter_select
+from bot_bottle.cli.tui import _filter_items, filter_multiselect, filter_select
 
 
 class TestFilterItems(unittest.TestCase):
@@ -46,5 +46,16 @@ class TestFilterSelectEmptyItems(unittest.TestCase):
         self.assertIsNone(result)
 
 
+class TestFilterMultiselectEmptyItems(unittest.TestCase):
+    def test_returns_empty_list_for_empty_items(self):
+        # No TTY needed — short-circuits before opening tty.
+        result = filter_multiselect([], title="Select", tty_path="/dev/null")
+        self.assertEqual([], result)
+
+    def test_returns_none_when_tty_unavailable(self):
+        result = filter_multiselect(["a", "b"], tty_path="/nonexistent/tty")
+        self.assertIsNone(result)
+
+
 if __name__ == "__main__":
     unittest.main()