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/contrib/pi/agent_provider.py

@@ -232,7 +232,7 @@ class PiAgentProvider(AgentProvider):
     def provision_skills(self, plan: "BottlePlan", bottle: "Bottle") -> None:
         from ...backend.util import host_skill_dir
 
-        agent = plan.spec.loaded_manifest.agent
+        agent = plan.manifest.agent
         if not agent.skills:
             return
         skills_dir = _skills_dir(plan.guest_home)