feat(agent): prune non-coding skill categories from the prompt index in coding posture

Unlike toolsets (where stripping fought explicit user opt-ins), pruning
the skill index is discovery-only: skills_list still returns the full
catalog and skill_view loads anything. Hiding cooking/social-media/
smart-home/etc. from the manifest while pairing on code reduces prompt
bloat and attention dilution without removing any capability.

- ContextProfile.hidden_skill_categories: deny-list on the profile
  (unknown/custom categories stay visible); coding profile hides the
  clearly-non-coding set, keeps coding-adjacent ones (github, devops,
  data-science, diagramming, research, security, ...).
- build_skills_system_prompt(hidden_categories=...): filters whole
  top-level categories (nested ones via their parent), participates in
  the LRU cache key, and appends a disclosure note so the model knows
  the full catalog exists.
- system_prompt wires it through coding_hidden_skill_categories();
  applies in every coding-posture mode (auto/focus/on), no-op otherwise.

tests/agent/test_coding_context.py

@@ -202,6 +202,21 @@ class TestProfiles:
         assert cc.GENERAL_PROFILE.toolset is None
         assert cc.GENERAL_PROFILE.guidance == ""
 
+    def test_skill_pruning_scoped_to_coding_posture(self, tmp_path):
+        # Coding posture hides clearly-non-coding categories; coding-adjacent
+        # ones stay visible (deny-list semantics).
+        _git_init(tmp_path)
+        coding = cc.resolve_runtime_mode(platform="cli", cwd=tmp_path, config={})
+        hidden = coding.hidden_skill_categories()
+        assert "social-media" in hidden and "smart-home" in hidden
+        for kept in ("github", "devops", "software-development", "data-science"):
+            assert kept not in hidden
+        # General posture hides nothing.
+        general = cc.resolve_runtime_mode(
+            platform="telegram", cwd=tmp_path, config={}
+        )
+        assert general.hidden_skill_categories() == frozenset()
+
 
 # ── detection signals ───────────────────────────────────────────────────────