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_docker_provision_git_user.py

@@ -21,7 +21,7 @@ from bot_bottle.backend import Bottle, BottleSpec, ExecResult
 from bot_bottle.backend.docker.bottle_plan import DockerBottlePlan
 from bot_bottle.egress import EgressPlan
 from bot_bottle.git_gate import GitGatePlan
-from bot_bottle.manifest import Manifest, ManifestIndex
+from bot_bottle.manifest import ManifestIndex
 
 
 class _Provider(AgentProvider):