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/agent_provider.py

@@ -240,7 +240,7 @@ class AgentProvider(ABC):
         BottleBackend.provision_workspace against the running bottle."""
         from .log import info
 
-        manifest_bottle = plan.spec.manifest.bottle
+        manifest_bottle = plan.spec.loaded_manifest.bottle
         if manifest_bottle.git:
             from .git_gate import GIT_GATE_HOSTNAME, git_gate_render_gitconfig
             gate_host = getattr(plan, "git_gate_insteadof_host", GIT_GATE_HOSTNAME)