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/macos_container/resolve_plan.py

@@ -30,7 +30,7 @@ def resolve_plan(
     resolved_env: ResolvedEnv,
     agent_provision_plan: AgentProvisionPlan,
     egress_plan: EgressPlan,
-    supervise_plan: SupervisePlan | None,
+    supervise_plan: SupervisePlan,
     git_gate_plan: GitGatePlan,
     stage_dir: Path,
 ) -> MacosContainerBottlePlan: