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/smolmachines/backend.py

@@ -18,6 +18,7 @@ from ...egress import EgressPlan
 from ...env import ResolvedEnv
 from ...git_gate import GitGatePlan
 from ...supervise import SupervisePlan
+from ...manifest import Manifest
 from .. import ActiveAgent, BottleBackend, BottleSpec
 from . import cleanup as _cleanup
 from . import enumerate as _enumerate
@@ -55,6 +56,7 @@ class SmolmachinesBottleBackend(
         self,
         spec: BottleSpec,
         *,
+        manifest: Manifest,
         slug: str,
         resolved_env: ResolvedEnv,
         agent_provision_plan: AgentProvisionPlan,
@@ -65,6 +67,7 @@ class SmolmachinesBottleBackend(
     ) -> SmolmachinesBottlePlan:
         return _resolve_plan.resolve_plan(
             spec,
+            manifest=manifest,
             slug=slug,
             resolved_env=resolved_env,
             agent_provision_plan=agent_provision_plan,

bot_bottle/backend/smolmachines/launch.py

@@ -130,7 +130,7 @@ def _teardown_smolmachines(
     except BaseException as exc:  # noqa: W0718 — teardown must not fail
         teardown_exc = exc
         warn(f"smolmachines teardown failed: {exc!r}")
-    bottle = plan.spec.loaded_manifest.bottle
+    bottle = plan.manifest.bottle
     revoke_git_gate_provisioned_keys(bottle, git_gate_state_dir(plan.slug))
     if teardown_exc is not None:
         raise teardown_exc

bot_bottle/backend/smolmachines/resolve_plan.py

@@ -13,6 +13,7 @@ from __future__ import annotations
 from pathlib import Path
 
 from .. import BottleSpec
+from ...manifest import Manifest
 from ...env import ResolvedEnv
 from ...agent_provider import AgentProvisionPlan
 from ...egress import EgressPlan
@@ -46,6 +47,7 @@ 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,
@@ -67,6 +69,7 @@ def resolve_plan(
 
     return SmolmachinesBottlePlan(
         spec=spec,
+        manifest=manifest,
         stage_dir=stage_dir,
         slug=slug,
         bundle_subnet=subnet,