Remove the supervise flag; supervise every bottle

Issue #249: in practice the per-bottle `supervise` flag was never
turned off — all bottles should be supervised. Remove the manifest
flag and make the supervise sidecar unconditional, mirroring egress.

- Reject `supervise:` as a removed bottle key with a migration hint.
- Drop the `supervise` field from ManifestBottle and the extends merge.
- prepare_supervise always returns a SupervisePlan; the plan type is
  now non-optional and the per-backend `is None` guards are gone, so
  the supervise daemon, current-config mount, aliases, and MCP
  registration always render.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01YcU7nerbg8cVj9R4EkpfLJ

bot_bottle/backend/__init__.py

@@ -102,7 +102,7 @@ class BottlePlan(ABC):
         over a published host port)."""
         return "git"
     egress_plan: EgressPlan
-    supervise_plan: SupervisePlan | None
+    supervise_plan: SupervisePlan
     agent_provision: AgentProvisionPlan
 
     @property
@@ -332,7 +332,7 @@ class BottleBackend(ABC, Generic[PlanT, CleanupT]):
         )
         agent_provision_plan = merge_provision_env_vars(agent_provision_plan)
         egress_plan = prepare_egress(manifest_bottle, slug, agent_provision_plan)
-        supervise_plan = prepare_supervise(manifest_bottle, slug)
+        supervise_plan = prepare_supervise(slug)
         git_gate_plan = prepare_git_gate(manifest_bottle, slug)
 
         return self._resolve_plan(
@@ -405,7 +405,7 @@ class BottleBackend(ABC, Generic[PlanT, CleanupT]):
                       agent_provision_plan: AgentProvisionPlan,
                       egress_plan: EgressPlan,
                       git_gate_plan: GitGatePlan,
-                      supervise_plan: SupervisePlan | None,
+                      supervise_plan: SupervisePlan,
                       stage_dir: Path) -> PlanT:
         """Backend-specific plan resolution: image/container names,
         env-file, prompt-file, proxy plan, runtime detection. Called by