fix(skill): re-inject skill index into local-agent system prompt

The contributor's original PR (#1917) appended an ``Available Skills``
index to the system prompt before the LLM saw the user message, so the
LLM could decide whether to activate a skill. ``7145447b`` removed the
text-marker activation flow and, together with it, the entire system
prompt injection — but the Tool Call replacement only put the available
skills inside the ``activate`` tool's description. In practice the LLM
ignores tool descriptions for selection and goes straight to native
tools, so user-visible skill activation silently broke.

Restore the injection, adapted for the Tool Call era:

- SkillManager regains ``get_skill_index(bound_skills)`` and
  ``build_skill_aware_prompt_addition(bound_skills)``. The addendum
  carries only ``name (display_name): description`` for each
  pipeline-visible skill plus one instruction line pointing at the
  ``activate`` tool. No SKILL.md contents — KV cache stays clean
- PreProcessor appends the addendum to the first system message (or
  inserts a new one) of ``query.prompt.messages`` for the local-agent
  runner. Handles plain-string and ContentElement[] bodies. Skips
  cleanly when no skills are visible
- 3 new test_preproc cases: injection happens, bound-skills subset
  honoured, empty addendum touches nothing. 280 passed

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/unit_tests/test_preproc.py

@@ -132,3 +132,73 @@ async def test_preproc_disables_skill_authoring_tools_when_skill_service_missing
 
     assert result.result_type == entities_module.ResultType.CONTINUE
     app.tool_mgr.get_all_tools.assert_awaited_once_with(None, None, include_skill_authoring=False)
+
+
+@pytest.mark.asyncio
+async def test_preproc_injects_skill_index_into_system_prompt():
+    """The Tool Call activation pattern still needs the LLM to know which
+    skills exist. PreProcessor must append the SkillManager's index
+    addendum to the first system message."""
+    preproc_module, entities_module = _import_preproc_modules()
+
+    app = _make_app(skill_service=SimpleNamespace())
+    addendum = '\n\nAvailable Skills:\n- demo (demo): Demo skill.\n\nCall activate ...'
+    app.skill_mgr.build_skill_aware_prompt_addition = Mock(return_value=addendum)
+
+    query = _make_query()
+    result = await stage_process_capture(preproc_module, app, query)
+
+    assert result.result_type == entities_module.ResultType.CONTINUE
+    app.skill_mgr.build_skill_aware_prompt_addition.assert_called_once_with(bound_skills=None)
+    head = query.prompt.messages[0]
+    assert head.role == 'system'
+    assert head.content.endswith(addendum)
+
+
+@pytest.mark.asyncio
+async def test_preproc_respects_pipeline_bound_skills_subset():
+    """When ``enable_all_skills`` is false the bound list is passed through
+    so the addendum only mentions skills allowed for this pipeline."""
+    preproc_module, entities_module = _import_preproc_modules()
+
+    app = _make_app(skill_service=SimpleNamespace())
+    app.pipeline_service.get_pipeline = AsyncMock(
+        return_value={
+            'extensions_preferences': {
+                'enable_all_skills': False,
+                'skills': ['only-this'],
+            }
+        }
+    )
+    app.skill_mgr.build_skill_aware_prompt_addition = Mock(return_value='')
+
+    query = _make_query()
+    result = await stage_process_capture(preproc_module, app, query)
+
+    assert result.result_type == entities_module.ResultType.CONTINUE
+    app.skill_mgr.build_skill_aware_prompt_addition.assert_called_once_with(bound_skills=['only-this'])
+    assert query.variables.get('_pipeline_bound_skills') == ['only-this']
+
+
+@pytest.mark.asyncio
+async def test_preproc_skips_injection_when_addendum_is_empty():
+    """No visible skills → system prompt is left untouched (no
+    ``Available Skills`` block appended)."""
+    preproc_module, entities_module = _import_preproc_modules()
+
+    app = _make_app(skill_service=SimpleNamespace())
+    app.skill_mgr.build_skill_aware_prompt_addition = Mock(return_value='')
+
+    query = _make_query()
+    result = await stage_process_capture(preproc_module, app, query)
+
+    assert result.result_type == entities_module.ResultType.CONTINUE
+    if query.prompt and query.prompt.messages:
+        assert 'Available Skills' not in (query.prompt.messages[0].content or '')
+
+
+async def stage_process_capture(preproc_module, app, query):
+    """Run PreProcessor.process and return the result while keeping ``query``
+    accessible to the assertions (process mutates query in place)."""
+    stage = preproc_module.PreProcessor(app)
+    return await stage.process(query, 'PreProcessor')