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_contrib_codex_provider.py

@@ -50,11 +50,8 @@ def _plan(
     agent_prompt: str = "",
     skills: list[str] | None = None,
     agent_provision: AgentProvisionPlan | None = None,
-    supervise: bool = False,
 ) -> DockerBottlePlan:
     bottle_json: dict = {"agent_provider": {"template": "codex"}}  # type: ignore
-    if supervise:
-        bottle_json["supervise"] = True
     index = ManifestIndex.from_json_obj({
         "bottles": {"dev": bottle_json},
         "agents": {
@@ -70,13 +67,11 @@ def _plan(
         manifest=index, agent_name="demo",
         copy_cwd=False, user_cwd="/tmp/x",
     )
-    supervise_plan = None
-    if supervise:
-        supervise_plan = SupervisePlan(
-            slug="demo-abc12",
-            queue_dir=Path("/tmp/queue"),
-            current_config_dir=Path("/tmp/current-config"),
-        )
+    supervise_plan = SupervisePlan(
+        slug="demo-abc12",
+        queue_dir=Path("/tmp/queue"),
+        current_config_dir=Path("/tmp/current-config"),
+    )
     return DockerBottlePlan(
         spec=spec,
         manifest=manifest,
@@ -277,17 +272,10 @@ class TestCodexProvision(unittest.TestCase):
 
 
 class TestCodexSuperviseMcp(unittest.TestCase):
-    def test_noop_when_supervise_disabled(self):
-        bottle = _make_bottle()
-        CodexAgentProvider().provision_supervise_mcp(
-            _plan(supervise=False), bottle, _URL,
-        )
-        bottle.exec.assert_not_called()
-
     def test_runs_codex_mcp_add_as_node(self):
         bottle = _make_bottle()
         CodexAgentProvider().provision_supervise_mcp(
-            _plan(supervise=True), bottle, _URL,
+            _plan(), bottle, _URL,
         )
         bottle.exec.assert_called_once()
         script = bottle.exec.call_args.args[0]
@@ -302,7 +290,7 @@ class TestCodexSuperviseMcp(unittest.TestCase):
             exec_result=ExecResult(returncode=1, stdout="", stderr="boom"),
         )
         CodexAgentProvider().provision_supervise_mcp(
-            _plan(supervise=True), bottle, _URL,
+            _plan(), bottle, _URL,
         )