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/resolve_common.py

@@ -92,11 +92,9 @@ def prepare_egress(
     return Egress().prepare(bottle, slug, egress_dir, provision.egress_routes)
 
 
-def prepare_supervise(bottle: ManifestBottle, slug: str) -> SupervisePlan | None:
-    """Prepare the supervise sidecar state dir. Returns None when
-    bottle.supervise is falsy."""
-    if not bottle.supervise:
-        return None
+def prepare_supervise(slug: str) -> SupervisePlan:
+    """Prepare the supervise sidecar state dir. Every bottle is
+    supervised (issue #249), so this always returns a plan."""
     supervise_dir = supervise_state_dir(slug)
     supervise_dir.mkdir(parents=True, exist_ok=True)
     return Supervise().prepare(slug, supervise_dir)