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/docker/launch.py

@@ -75,7 +75,7 @@ def launch(
     Teardown on exit."""
     stack = ExitStack()
 
-    _bottle_for_revoke = plan.spec.manifest.bottle
+    _bottle_for_revoke = plan.spec.loaded_manifest.bottle
     _git_gate_dir_for_revoke = git_gate_state_dir(plan.slug)
 
     def teardown() -> None: