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/smolmachines/launch.py

@@ -130,7 +130,7 @@ def _teardown_smolmachines(
     except BaseException as exc:  # noqa: W0718 — teardown must not fail
         teardown_exc = exc
         warn(f"smolmachines teardown failed: {exc!r}")
-    bottle = plan.spec.manifest.bottle
+    bottle = plan.spec.loaded_manifest.bottle
     revoke_git_gate_provisioned_keys(bottle, git_gate_state_dir(plan.slug))
     if teardown_exc is not None:
         raise teardown_exc