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_manifest_agent_git_user.py

@@ -116,7 +116,6 @@ class TestAgentGitUserOverlay(unittest.TestCase):
         idx = ManifestIndex.from_json_obj({
             "bottles": {"dev": {
                 "env": {"FOO": "bar"},
-                "supervise": True,
                 "git-gate": {"user": {"name": "B"}},
             }},
             "agents": {"impl": {
@@ -127,7 +126,6 @@ class TestAgentGitUserOverlay(unittest.TestCase):
         b = idx.load_for_agent("impl").bottle
         self.assertEqual("a", b.git_user.name)
         self.assertEqual({"FOO": "bar"}, dict(b.env))
-        self.assertTrue(b.supervise)
 
 
 class TestAgentGitUserRejections(unittest.TestCase):