refactor(manifest): split Manifest into ManifestIndex + Manifest single-value type

Manifest now holds exactly one agent and one effective bottle (with
git_user overlay already applied). The old multi-agent/bottle
collection is renamed ManifestIndex. BottleSpec.manifest starts as
ManifestIndex from the CLI and becomes Manifest after _validate()
calls load_for_agent(); all provisioning code downstream reads
spec.manifest.agent / spec.manifest.bottle instead of indexing by name.

tests/unit/test_manifest_extends.py

@@ -12,7 +12,7 @@ from __future__ import annotations
 
 import unittest
 
-from bot_bottle.manifest import ManifestError, Manifest
+from bot_bottle.manifest import ManifestError, ManifestIndex
 
 
 def _error_message(callable_, *args, **kwargs) -> str:  # type: ignore
@@ -28,7 +28,7 @@ def _build(**bottles) -> Manifest:  # type: ignore
     """Build a manifest with the given bottles and one trivial agent
     referencing the first bottle (so the manifest is valid)."""
     first = next(iter(bottles))
-    return Manifest.from_json_obj({
+    return ManifestIndex.from_json_obj({
         "bottles": bottles,
         "agents": {
             "demo": {"skills": [], "prompt": "", "bottle": first},