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/unit/test_docker_provision_git_user.py

@@ -21,6 +21,7 @@ from bot_bottle.backend import Bottle, BottleSpec, ExecResult
 from bot_bottle.backend.docker.bottle_plan import DockerBottlePlan
 from bot_bottle.egress import EgressPlan
 from bot_bottle.git_gate import GitGatePlan
+from bot_bottle.supervise import SupervisePlan
 from bot_bottle.manifest import ManifestIndex
 
 
@@ -79,7 +80,11 @@ def _plan(*, git_user: dict | None = None,  # type: ignore
             routes=(),
             token_env_map={},
         ),
-        supervise_plan=None,
+        supervise_plan=SupervisePlan(
+            slug="demo-abc12",
+            queue_dir=Path("/tmp/queue"),
+            current_config_dir=Path("/tmp/current-config"),
+        ),
         use_runsc=False,
         agent_provision=AgentProvisionPlan(
             template="claude",