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/bottle_state.py

@@ -112,6 +112,10 @@ class BottleMetadata:
     backend: str = ""
     label: str = ""
     color: str = ""
+    # Ordered bottle names selected at launch (issue #269). Empty tuple
+    # for state dirs written before this change; resume falls back to
+    # the agent's `bottle:` field in that case.
+    bottle_names: tuple[str, ...] = ()
 
 
 def metadata_path(identity: str) -> Path:
@@ -139,6 +143,10 @@ def read_metadata(identity: str) -> BottleMetadata | None:
     if not isinstance(raw, dict):
         return None
     raw_typed = cast(dict[str, object], raw)
+    raw_bottle_names = raw_typed.get("bottle_names", [])
+    bottle_names: tuple[str, ...] = ()
+    if isinstance(raw_bottle_names, list):
+        bottle_names = tuple(str(n) for n in raw_bottle_names if isinstance(n, str))
     return BottleMetadata(
         identity=str(raw_typed.get("identity", identity)),
         agent_name=str(raw_typed.get("agent_name", "")),
@@ -149,6 +157,7 @@ def read_metadata(identity: str) -> BottleMetadata | None:
         backend=str(raw_typed.get("backend", "")),
         label=str(raw_typed.get("label", "")),
         color=str(raw_typed.get("color", "")),
+        bottle_names=bottle_names,
     )