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/manifest_extends.py

@@ -134,9 +134,6 @@ def _merge_bottles(
         if "agent_provider" in child_raw
         else parent.agent_provider
     )
-    merged_supervise = (
-        child.supervise if "supervise" in child_raw else parent.supervise
-    )
     validate_egress_routes(name, merged_egress.routes)
 
     return ManifestBottle(
@@ -145,7 +142,6 @@ def _merge_bottles(
         git=merged_git,
         git_user=merged_git_user,
         egress=merged_egress,
-        supervise=merged_supervise,
     )