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_extends.py

@@ -42,38 +42,26 @@ class TestExtendsBasic(unittest.TestCase):
         # same way they did before the resolver landed.
         m = _build(dev={
             "env": {"FOO": "bar"},
-            "supervise": True,
         })
         b = m.bottles["dev"]
         self.assertEqual({"FOO": "bar"}, dict(b.env))
-        self.assertTrue(b.supervise)
 
     def test_child_inherits_parent_fields_unchanged(self):
         m = _build(
             base={
                 "env": {"BASE": "1"},
-                "supervise": True,
             },
             child={"extends": "base"},
         )
         c = m.bottles["child"]
         self.assertEqual({"BASE": "1"}, dict(c.env))
-        self.assertTrue(c.supervise)
-
-    def test_child_overrides_supervise_scalar(self):
-        m = _build(
-            base={"supervise": True},
-            off={"extends": "base", "supervise": False},
-        )
-        self.assertTrue(m.bottles["base"].supervise)
-        self.assertFalse(m.bottles["off"].supervise)
 
     def test_parent_resolved_once_for_multiple_children(self):
         # Two children sharing one parent: both inherit; the parent
         # is resolved once + cached. (Cache behavior is internal; we
         # observe correctness on both children.)
         m = _build(
-            base={"env": {"BASE": "1"}, "supervise": True},
+            base={"env": {"BASE": "1"}},
             a={"extends": "base", "env": {"A": "1"}},
             b={"extends": "base", "env": {"B": "1"}},
         )
@@ -366,7 +354,6 @@ class TestExtendsChain(unittest.TestCase):
         m = _build(
             grandparent={
                 "env": {"GP": "1"},
-                "supervise": True,
             },
             parent={
                 "extends": "grandparent",
@@ -381,8 +368,6 @@ class TestExtendsChain(unittest.TestCase):
             {"GP": "1", "P": "1", "C": "1"},
             dict(m.bottles["child"].env),
         )
-        # supervise threads through unchanged.
-        self.assertTrue(m.bottles["child"].supervise)
 
     def test_intermediate_can_override(self):
         m = _build(