fix(agent): demote non-coding skill categories to names-only — never hide skills (#44342)

Real-world failure with the original index pruning: under the default auto
posture, an agent-created ops skill in a demoted category vanished from the
prompt's skill index mid-project, and the agent silently fell back to a
stale sibling skill instead. The "discovery-only" premise didn't hold —
models do not reach for skills_list to rediscover what the index stops
showing them, and agent-created skills are the model's accumulated project
memory (runbooks, pitfalls, operating rules).

Gating pruning behind the opt-in focus mode was the wrong fix too: users
opening a worktree don't know the config exists, so the index-noise win
would effectively never ship.

Instead, the coding posture now DEMOTES non-coding categories rather than
hiding them: each demoted category renders as a single names-only line
("gaming [names only]: allthemons10-ops, mc-backup") with a footer note
explaining the omitted descriptions. Every skill name stays in the prompt,
so memory-anchored recall ("load <name>") keeps working in every mode,
while the description noise is still cut. Applies in auto/on/focus alike;
the general posture demotes nothing. Deny-list semantics unchanged —
unknown/custom categories and coding-adjacent ones keep full entries.

API renamed to match the honest semantics: hidden_skill_categories →
compact_skill_categories, build_skills_system_prompt(hidden_categories=) →
compact_categories=.

agent/system_prompt.py

@@ -191,21 +191,22 @@ def build_system_prompt_parts(agent: Any, system_message: Optional[str] = None)
             )
             if toolset
         }
-        # Coding posture prunes non-coding skill categories from the index
-        # (discovery-only — skills_list/skill_view still reach everything).
-        _hidden_cats = frozenset()
+        # Coding posture demotes non-coding skill categories to names-only in
+        # the index (never hidden — skill_view/skills_list reach everything,
+        # and every name stays visible for memory-anchored recall).
+        _compact_cats = frozenset()
         try:
-            from agent.coding_context import coding_hidden_skill_categories
+            from agent.coding_context import coding_compact_skill_categories
 
-            _hidden_cats = coding_hidden_skill_categories(
+            _compact_cats = coding_compact_skill_categories(
                 platform=agent.platform, cwd=resolve_context_cwd()
             )
         except Exception:
-            _hidden_cats = frozenset()
+            _compact_cats = frozenset()
         skills_prompt = _r.build_skills_system_prompt(
             available_tools=agent.valid_tool_names,
             available_toolsets=avail_toolsets,
-            hidden_categories=_hidden_cats or None,
+            compact_categories=_compact_cats or None,
         )
     else:
         skills_prompt = ""