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.

bot_bottle/backend/resolve_common.py

@@ -69,8 +69,7 @@ def write_launch_metadata(
 def prepare_agent_state_dir(slug: str, spec: BottleSpec) -> tuple[Path, Path]:
     """Create the agent state subdir, write the prompt file.
     Returns (agent_dir, prompt_file)."""
-    manifest = spec.manifest  # type: ignore[assignment]
-    agent = manifest.agent
+    agent = spec.loaded_manifest.agent
     agent_dir = agent_state_dir(slug)
     agent_dir.mkdir(parents=True, exist_ok=True)
     prompt_file = agent_dir / "prompt.txt"