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/agent_provider.py

@@ -207,8 +207,7 @@ class AgentProvider(ABC):
     ) -> None:
         """Register the per-bottle supervise sidecar as an MCP server
         in the provider's in-guest config. Called by the backend after
-        the supervise sidecar is reachable. No-op when
-        `plan.supervise_plan is None`."""
+        the supervise sidecar is reachable."""
 
     def provision_ca(self, bottle: "Bottle", plan: "BottlePlan") -> None:
         """Install the egress MITM CA into the agent's trust store.