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_smolmachines_provision.py

@@ -86,7 +86,6 @@ def _plan(
     stage_dir: Path | None = None,
     egress_routes: tuple[EgressRoute, ...] = (),
     egress_ca_path: Path = Path(),
-    supervise: bool = False,
     bundle_ip: str = "192.168.50.2",
     agent_git_gate_host: str = "127.0.0.1:55555",
     agent_supervise_url: str = "http://127.0.0.1:55556/",
@@ -108,8 +107,6 @@ def _plan(
         git_gate_json["user"] = git_user
     if git_gate_json:
         bottle_json["git-gate"] = git_gate_json
-    if supervise:
-        bottle_json["supervise"] = True
     index = ManifestIndex.from_json_obj({
         "bottles": {"dev": bottle_json},
         "agents": {
@@ -127,13 +124,11 @@ def _plan(
         copy_cwd=copy_cwd,
         user_cwd=user_cwd,
     )
-    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 SmolmachinesBottlePlan(
         spec=spec,
         manifest=manifest,
@@ -405,7 +400,7 @@ class TestBundleLaunchSpec(unittest.TestCase):
         spec = _bundle_launch_spec(plan, "net", "127.0.0.16")
 
         self.assertEqual(
-            "egress,git-gate,git-http",
+            "egress,supervise,git-gate,git-http",
             spec.daemons_csv,
         )
         self.assertIn(9420, spec.ports_to_publish)