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/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.loaded_manifest.bottle
+        manifest_bottle = plan.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)