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/smolmachines/sidecar_bundle.py

@@ -68,7 +68,8 @@ class BundleLaunchSpec:
     image: str = SIDECAR_BUNDLE_IMAGE
     # Daemon subset CSV for BOT_BOTTLE_SIDECAR_DAEMONS. The
     # supervisor inside the bundle reads it to skip
-    # bottle-irrelevant daemons (e.g. supervise=False bottles).
+    # bottle-irrelevant daemons (e.g. git-gate when a bottle
+    # declares no upstreams).
     daemons_csv: str = "egress"
     # Plain "KEY=VALUE" strings + "KEY" bare names (the bare-name
     # form inherits the value from the docker-run subprocess env,