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

tests/integration/test_sidecar_bundle_compose.py

@@ -27,14 +27,13 @@ from tests._docker import skip_unless_docker
 
 
 def _manifest() -> ManifestIndex:
-    """Bottle with supervise on so the bundle exercises egress +
-    supervise. Git is off because a meaningful git-gate test needs
-    a real upstream and SSH keys — out of scope for a bundle smoke."""
+    """Minimal bottle so the bundle exercises egress + supervise
+    (every bottle is supervised, issue #249). Git is off because a
+    meaningful git-gate test needs a real upstream and SSH keys —
+    out of scope for a bundle smoke."""
     return ManifestIndex.from_json_obj({
         "bottles": {
-            "dev": {
-                "supervise": True,
-            },
+            "dev": {},
         },
         "agents": {
             "demo": {"skills": [], "prompt": "", "bottle": "dev"},