fix(librarian): quiet healthy-search coverage notes, tag clearing, strict result pairing

Phase A review minors:

- Coverage note: source_status (when present) is now used exclusively;
  the source_counts-absence fallback only considers the optional legs
  the request explicitly enabled (web/documents/volatile). library-desk
  computes source_counts from the final top-N fused results only, so
  absence of the always-on vector/graph legs is normal ranking behavior
  - the old heuristic warned on virtually every healthy search
- update_wiki_page: the empty-list tags sentinel (leave unchanged) made
  clearing all tags impossible; pass exactly ["__CLEAR__"] to send an
  empty tag list, documented in the docstring for the local model
- Text-delegation parallel fallback: zip(..., strict=True) with an
  explicit count-mismatch guard so results can never be silently
  attributed to the wrong agent

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QbFZyDvYksazX6nYQYZ67L

tests/agents/librarian/test_tools.py

@@ -14,11 +14,14 @@ from src.agents.librarian.client import (
     ContentExtractionResult,
     WebSearchResponse,
     WebSearchResult,
+    WikiPage,
 )
 from src.agents.librarian.tools import (
+    CLEAR_TAGS_SENTINEL,
     read_url,
     read_urls_batch,
     search_web,
+    update_wiki_page,
 )
 
 
@@ -427,3 +430,59 @@ class TestWebSearchModels:
         assert response.total_urls == 2
         assert response.successful == 1
         assert response.failed == 1
+
+
+# ============================================================================
+# Wiki Update Tests (tag sentinel behavior)
+# ============================================================================
+
+@pytest.mark.unit
+class TestUpdateWikiPageTagSentinels:
+    """Empty list leaves tags unchanged; the clear sentinel empties them."""
+
+    def _page(self, tags: list[str]) -> WikiPage:
+        return WikiPage(id=42, path="test/page", title="Test Page", tags=tags)
+
+    def _patched_client(self, mock_client):
+        patcher = patch("src.agents.librarian.tools.LibraryDeskClient")
+        mock_client_class = patcher.start()
+        mock_client_class.return_value.__aenter__.return_value = mock_client
+        mock_client_class.return_value.__aexit__.return_value = None
+        return patcher
+
+    @pytest.mark.asyncio
+    async def test_empty_tags_means_leave_unchanged(self, mock_client):
+        mock_client.update_wiki_page.return_value = self._page(["existing"])
+        patcher = self._patched_client(mock_client)
+        try:
+            await update_wiki_page(42, content="new content")
+        finally:
+            patcher.stop()
+
+        _, kwargs = mock_client.update_wiki_page.call_args
+        assert kwargs["tags"] is None
+
+    @pytest.mark.asyncio
+    async def test_clear_sentinel_sends_empty_tag_list(self, mock_client):
+        mock_client.update_wiki_page.return_value = self._page([])
+        patcher = self._patched_client(mock_client)
+        try:
+            result = await update_wiki_page(42, tags=[CLEAR_TAGS_SENTINEL])
+        finally:
+            patcher.stop()
+
+        _, kwargs = mock_client.update_wiki_page.call_args
+        assert kwargs["tags"] == []
+        assert "tags (cleared)" in result
+
+    @pytest.mark.asyncio
+    async def test_real_tags_are_passed_through(self, mock_client):
+        mock_client.update_wiki_page.return_value = self._page(["a", "b"])
+        patcher = self._patched_client(mock_client)
+        try:
+            await update_wiki_page(42, tags=["a", "b"])
+        finally:
+            patcher.stop()
+
+        _, kwargs = mock_client.update_wiki_page.call_args
+        assert kwargs["tags"] == ["a", "b"]