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_plan_print_parity.py

@@ -19,6 +19,7 @@ from bot_bottle.backend.docker.bottle_plan import DockerBottlePlan
 from bot_bottle.backend.smolmachines.bottle_plan import SmolmachinesBottlePlan
 from bot_bottle.egress import EgressPlan, EgressRoute
 from bot_bottle.git_gate import GitGatePlan, GitGateUpstream
+from bot_bottle.supervise import SupervisePlan
 from bot_bottle.manifest import Manifest, ManifestIndex
 
 
@@ -77,6 +78,15 @@ def _egress_plan(tmp: str) -> EgressPlan:
     )
 
 
+def _supervise_plan(tmp: str) -> SupervisePlan:
+    stage = Path(tmp)
+    return SupervisePlan(
+        slug="test-00001",
+        queue_dir=stage / "supervise" / "queue",
+        current_config_dir=stage / "supervise" / "current-config",
+    )
+
+
 def _agent_provision(tmp: str) -> AgentProvisionPlan:
     return AgentProvisionPlan(
         template="claude",
@@ -99,7 +109,7 @@ def _docker_plan(spec: BottleSpec, manifest: Manifest, tmp: str) -> DockerBottle
         stage_dir=stage,
         git_gate_plan=_git_gate_plan(tmp),
         egress_plan=_egress_plan(tmp),
-        supervise_plan=None,
+        supervise_plan=_supervise_plan(tmp),
         agent_provision=_agent_provision(tmp),
         slug="test-00001",
         forwarded_env={},
@@ -115,7 +125,7 @@ def _smolmachines_plan(spec: BottleSpec, manifest: Manifest, tmp: str) -> Smolma
         stage_dir=stage,
         git_gate_plan=_git_gate_plan(tmp),
         egress_plan=_egress_plan(tmp),
-        supervise_plan=None,
+        supervise_plan=_supervise_plan(tmp),
         agent_provision=_agent_provision(tmp),
         slug="test-00001",
         bundle_subnet="10.99.0.0/24",