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/contrib/claude/agent_provider.py

@@ -291,8 +291,6 @@ class ClaudeAgentProvider(AgentProvider):
 
         Failure is logged but not fatal — the bottle still works without
         the entry; the operator can register it manually."""
-        if plan.supervise_plan is None:
-            return
         info(f"registering supervise MCP server in agent claude config → {supervise_url}")
         r = bottle.exec(
             f"claude mcp add --scope user --transport http "