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.