fix(types): add BottleSpec.loaded_manifest to satisfy pyright on union type

BottleSpec.manifest is ManifestIndex | Manifest (pre/post _validate()).
Downstream code always runs post-validate so it needs Manifest, but
pyright flagged every .agent/.bottle access. The new loaded_manifest
property asserts isinstance and returns Manifest, giving pyright a
narrowed type without scattering type: ignore everywhere.

Also remove unused Manifest imports from test files and annotate the
_index() helper in test_manifest_agent_git_user.

tests/unit/test_manifest_md_load.py

@@ -11,7 +11,7 @@ import textwrap
 import unittest
 from pathlib import Path
 
-from bot_bottle.manifest import ManifestError, Manifest, ManifestIndex
+from bot_bottle.manifest import ManifestError, ManifestIndex
 
 
 def _write(p: Path, text: str) -> None: