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).

agent/coding_context.py

@@ -38,12 +38,13 @@ session (deferred), the same contract as ``/skills install`` vs ``--now``.
 
 Activation (config ``agent.coding_context``):
 
-  * ``auto`` (default) — posture (brief + snapshot + names-only demotion of
-    non-coding skill categories) on an interactive coding surface sitting in
-    a code workspace (git repo or recognised project root). Prompt-only;
-    toolsets untouched, no skill is ever hidden.
+  * ``auto`` (default) — posture (brief + snapshot) on an interactive coding
+    surface sitting in a code workspace (git repo or recognised project root).
+    Prompt-only; toolsets and the skill index untouched.
   * ``focus`` — like ``auto``, but additionally collapses the toolset to the
-    ``coding`` set + enabled MCP servers. Explicit opt-in for a lean schema.
+    ``coding`` set + enabled MCP servers and demotes non-coding skill
+    categories to names-only in the prompt's skill index (no skill is ever
+    hidden). Explicit opt-in for a lean schema.
   * ``on`` — force the posture anywhere (incl. non-workspaces). Prompt-only.
   * ``off`` — disable entirely.
 """
@@ -214,8 +215,8 @@ class ContextProfile:
                        (extension seam; not yet consumed by the router).
     ``memory_policy``— memory namespace/weighting hint (extension seam).
     ``compact_skill_categories`` — skill categories DEMOTED to names-only in
-                       the system-prompt skill index while this posture is
-                       active. Never hidden: every skill name stays visible
+                       the system-prompt skill index under the opt-in ``focus``
+                       mode. Never hidden: every skill name stays visible
                        (so memory-anchored recall keeps working) — only the
                        descriptions are dropped to cut index noise. Deny-list
                        semantics so unknown/custom categories keep full
@@ -231,7 +232,7 @@ class ContextProfile:
 
 
 # Skill categories that are clearly not part of a coding workflow. Demoted to
-# names-only in the prompt's skill index while the coding posture is active
+# names-only in the prompt's skill index under the opt-in ``focus`` mode only
 # (deny-list — anything not listed here, incl. custom user categories, keeps
 # full entries). Coding-adjacent categories (devops, github, mcp,
 # data-science, diagramming, research, security, …) are intentionally absent.
@@ -438,15 +439,22 @@ class RuntimeMode:
     def compact_skill_categories(self) -> frozenset[str]:
         """Skill categories to demote to names-only in the prompt's skill index.
 
-        Demoted — never hidden. An earlier revision fully pruned these
-        categories from the index, which caused silent capability loss in a
-        real workflow: agent-created skills are the model's accumulated
-        project memory (server-ops runbooks, learned pitfalls, …), and models
-        do not reliably reach for ``skills_list`` to rediscover what the
-        index stopped showing them. Names-only keeps every skill loadable on
-        recall while still cutting the description noise from the index.
+        Gated on the opt-in ``focus`` mode, like the toolset collapse: the
+        default posture leaves the skill index untouched. Users who didn't ask
+        for a lean prompt keep full entries for every category — index changes
+        under ``auto`` proved too surprising in practice, even names-only ones
+        (a demoted description is information the model no longer weighs when
+        deciding what to load).
+
+        Demoted — never hidden — even under ``focus``. An earlier revision
+        fully pruned these categories from the index, which caused silent
+        capability loss in a real workflow: agent-created skills are the
+        model's accumulated project memory (server-ops runbooks, learned
+        pitfalls, …), and models do not reliably reach for ``skills_list`` to
+        rediscover what the index stopped showing them. Names-only keeps every
+        skill loadable on recall while still cutting the description noise.
         """
-        if not self.is_coding:
+        if not self.is_coding or self.config_mode != "focus":
             return frozenset()
         return frozenset(self.profile.compact_skill_categories)
 
@@ -534,9 +542,11 @@ def coding_compact_skill_categories(
 ) -> frozenset[str]:
     """Skill categories the active posture demotes to names-only in the index.
 
-    Empty outside the coding posture. Demoted — never hidden: every skill
-    name stays in the index and remains loadable via ``skill_view`` /
-    ``skills_list``; only descriptions are dropped.
+    Empty outside the coding posture and outside the opt-in ``focus`` mode —
+    the default posture never touches the skill index. Under ``focus``,
+    demoted — never hidden: every skill name stays in the index and remains
+    loadable via ``skill_view`` / ``skills_list``; only descriptions are
+    dropped.
     """
     return resolve_runtime_mode(
         platform=platform, cwd=cwd, config=config