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/docker/backend.py

@@ -70,7 +70,7 @@ class DockerBottleBackend(BottleBackend["DockerBottlePlan", "DockerBottleCleanup
         agent_provision_plan: AgentProvisionPlan,
         egress_plan: EgressPlan,
         git_gate_plan: GitGatePlan,
-        supervise_plan: SupervisePlan | None,
+        supervise_plan: SupervisePlan,
         stage_dir: Path,
     ) -> DockerBottlePlan:
         return _resolve_plan.resolve_plan(
@@ -94,8 +94,6 @@ class DockerBottleBackend(BottleBackend["DockerBottlePlan", "DockerBottleCleanup
         """Docker bottles reach the supervise sidecar via the
         compose-network alias `supervise:9100`. No per-bottle URL
         plumbing needed; the alias resolves inside the bridge."""
-        if plan.supervise_plan is None:
-            return ""
         return f"http://{SUPERVISE_HOSTNAME}:{SUPERVISE_PORT}/"
 
     def prepare_cleanup(self) -> DockerBottleCleanupPlan: