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/launch.py

@@ -130,12 +130,10 @@ def launch(
             mitmproxy_ca_host_path=egress_ca_host,
             mitmproxy_ca_cert_only_host_path=egress_ca_cert_only,
         )
-        supervise_plan = plan.supervise_plan
-        if supervise_plan is not None:
-            supervise_plan = dataclasses.replace(
-                supervise_plan,
-                internal_network=internal_network,
-            )
+        supervise_plan = dataclasses.replace(
+            plan.supervise_plan,
+            internal_network=internal_network,
+        )
         plan = dataclasses.replace(
             plan,
             git_gate_plan=git_gate_plan,