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.

bot_bottle/cli/resume.py

@@ -49,6 +49,7 @@ def cmd_resume(argv: list[str]) -> int:
         copy_cwd=metadata.copy_cwd,
         user_cwd=metadata.cwd or USER_CWD,
         identity=metadata.identity,
+        bottle_names=tuple(metadata.bottle_names),
     )
     backend_name = metadata.backend or None
     return _launch_bottle(