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/docker/resolve_plan.py

@@ -18,6 +18,7 @@ from .. import BottleSpec
 from ...env import ResolvedEnv
 from ...agent_provider import AgentProvisionPlan
 from ...egress import EgressPlan
+from ...manifest import Manifest
 from ...supervise import SupervisePlan
 from ...git_gate import GitGatePlan
 
@@ -31,6 +32,7 @@ def build_guest_env(resolved_env: ResolvedEnv) -> dict[str, str]:
 
 def resolve_plan(
     spec: BottleSpec,
+    manifest: Manifest,
     slug: str,
     resolved_env: ResolvedEnv,
     agent_provision_plan: AgentProvisionPlan,
@@ -48,6 +50,7 @@ def resolve_plan(
 
     return DockerBottlePlan(
         spec=spec,
+        manifest=manifest,
         stage_dir=stage_dir,
         slug=slug,
         forwarded_env=dict(resolved_env.forwarded),