refactor(types): move loaded manifest from BottleSpec to BottlePlan

BottleSpec.manifest was ManifestIndex | Manifest — a union encoding
two lifecycle stages in one field. The union was unjustifiable:
it forced a type-narrowing workaround (loaded_manifest property)
on every consumer.

Clean split:
- BottleSpec.manifest: ManifestIndex (always; CLI-supplied intent)
- BottlePlan.manifest: Manifest (always; loaded by _validate())

_validate() returns the loaded Manifest directly. prepare() passes
it to _resolve_plan(), which stores it on the plan. All provisioner
code now reads plan.manifest.agent / plan.manifest.bottle — no
union, no asserts, no type: ignore.

bot_bottle/backend/macos_container/launch.py

@@ -68,7 +68,7 @@ def launch(
 ) -> Generator[MacosContainerBottle, None, None]:
     """Build, run, provision, and yield an Apple Container bottle."""
     stack = ExitStack()
-    bottle_for_revoke = plan.spec.loaded_manifest.bottle
+    bottle_for_revoke = plan.manifest.bottle
     git_gate_dir_for_revoke = git_gate_state_dir(plan.slug)
 
     def teardown() -> None: