fix(agent): gate skill-index demotion behind the opt-in focus mode (#44387)

The coding posture's names-only demotion of non-coding skill categories
(#44342) applied under the default auto mode, silently changing the skill
index for every user in a git repo. Index changes must be opt-in: demotion
now only fires under agent.coding_context=focus, alongside the toolset
collapse. auto/on leave the skill index untouched; focus semantics are
unchanged (demoted, never hidden; deny-list keeps coding-adjacent and
custom categories at full entries).

tests/agent/test_coding_context.py

@@ -368,21 +368,27 @@ class TestProfiles:
         assert cc.GENERAL_PROFILE.toolset is None
         assert cc.GENERAL_PROFILE.guidance == ""
 
-    def test_skill_demotion_scoped_to_coding_posture(self, tmp_path):
-        # Coding posture demotes clearly-non-coding categories to names-only
-        # in the index (never hides them — agent-created skills are the
-        # model's project memory and must stay recallable by name).
-        # Coding-adjacent categories keep full entries (deny-list semantics).
+    def test_skill_demotion_gated_on_focus(self, tmp_path):
+        # Names-only demotion is opt-in via focus mode — the default (auto)
+        # and forced (on) postures leave the skill index untouched. Under
+        # focus, clearly-non-coding categories are demoted (never hidden) and
+        # coding-adjacent ones keep full entries (deny-list semantics).
         _git_init(tmp_path)
-        for raw in ("auto", "on", "focus"):
+        for raw in ("auto", "on"):
             mode = cc.resolve_runtime_mode(
                 platform="cli", cwd=tmp_path, config={"agent": {"coding_context": raw}}
             )
             assert mode.is_coding is True
-            compact = mode.compact_skill_categories()
-            assert "social-media" in compact and "smart-home" in compact
-            for kept in ("github", "devops", "software-development", "data-science"):
-                assert kept not in compact
+            assert mode.compact_skill_categories() == frozenset()
+        focus = cc.resolve_runtime_mode(
+            platform="cli", cwd=tmp_path,
+            config={"agent": {"coding_context": "focus"}},
+        )
+        assert focus.is_coding is True
+        compact = focus.compact_skill_categories()
+        assert "social-media" in compact and "smart-home" in compact
+        for kept in ("github", "devops", "software-development", "data-science"):
+            assert kept not in compact
         # General posture demotes nothing.
         general = cc.resolve_runtime_mode(platform="telegram", cwd=tmp_path, config={})
         assert general.compact_skill_categories() == frozenset()