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

src/agents/librarian/tools.py

@@ -55,8 +55,13 @@ def _coverage_note(
     enabled source leg contributed nothing, so outages stay visible to
     the model and the user instead of silently narrowing results.
 
-    Prefers the additive source_status/degraded contract when present;
-    falls back to inferring silent legs from source_counts.
+    When the additive source_status/degraded contract is present it is
+    authoritative and used EXCLUSIVELY - no count heuristics. Without
+    it, absence from source_counts is only inferred for the optional
+    legs this request explicitly enabled (web/documents/volatile);
+    the always-on wiki legs (vector/graph) are never inferred, because
+    source_counts only tallies the sources of the final top-N fused
+    results, so their absence is normal ranking behavior, not an outage.
     """
     if response.source_status:
         failed = sorted(
@@ -80,7 +85,9 @@ def _coverage_note(
         # Older library-desk without per-source reporting - nothing to infer
         return ""
 
-    expected = {"vector", "graph"}
+    # Only legs the request explicitly enabled; never vector/graph (their
+    # absence from the top-N counts is healthy, see docstring)
+    expected = set()
     if include_web:
         expected.add("web")
     if include_documents:
@@ -88,8 +95,8 @@ def _coverage_note(
     if include_volatile:
         expected.add("volatile")
 
-    # Normalize count keys to leg names (document/documents, wiki/vector)
-    aliases = {"document": "documents", "wiki": "vector"}
+    # Normalize count keys to leg names (document/documents)
+    aliases = {"document": "documents"}
     reported = {aliases.get(key, key) for key in response.source_counts}
     missing = sorted(expected - reported)
     if missing:
@@ -791,6 +798,9 @@ async def read_urls_batch(
 # Wiki Write Operations
 # ============================================================================
 
+CLEAR_TAGS_SENTINEL = "__CLEAR__"
+
+
 async def update_wiki_page(
     page_id: int,
     content: str = "",
@@ -817,7 +827,9 @@ async def update_wiki_page(
         page_id: ID of the page to update (get from search_wiki results)
         content: New markdown content (empty = leave unchanged)
         title: New title (empty = leave unchanged)
-        tags: New tag list, replaces existing tags (empty = leave unchanged)
+        tags: New tag list, replaces existing tags (empty = leave unchanged).
+            To remove ALL tags from a page, pass exactly ["__CLEAR__"]
+            (an empty list means "leave unchanged", not "clear")
         description: New description (empty = leave unchanged)
 
     Returns:
@@ -826,15 +838,20 @@ async def update_wiki_page(
     Examples:
         update_wiki_page(42, content="# Updated Content\\n\\nNew information here")
         update_wiki_page(42, tags=["projects", "devops"])  # Add to dossiers
+        update_wiki_page(42, tags=["__CLEAR__"])  # Remove all tags
         update_wiki_page(42, description="Updated description")
     """
+    # Empty list = leave unchanged; the explicit clear sentinel sends an
+    # empty tag list to the service, which replaces (clears) all tags.
+    clear_tags = tags == [CLEAR_TAGS_SENTINEL]
+
     try:
         async with LibraryDeskClient() as client:
             page = await client.update_wiki_page(
                 page_id=page_id,
                 content=content if content else None,
                 title=title if title else None,
-                tags=tags if tags else None,
+                tags=[] if clear_tags else (tags if tags else None),
                 description=description if description else None,
             )
 
@@ -844,7 +861,9 @@ async def update_wiki_page(
                 updated_fields.append("content")
             if title:
                 updated_fields.append("title")
-            if tags:
+            if clear_tags:
+                updated_fields.append("tags (cleared)")
+            elif tags:
                 updated_fields.append("tags")
             if description:
                 updated_fields.append("description")

src/responses/service.py

@@ -178,9 +178,24 @@ async def _handle_text_delegation(
             ]
             results = await asyncio.gather(*tasks, return_exceptions=True)
 
+            # asyncio.gather returns exactly one item per task; a length
+            # mismatch would mean results are attributed to the wrong
+            # agent, so fail loudly instead of mispairing silently.
+            if len(results) != len(matches):
+                logger.error(
+                    "delegation_result_count_mismatch",
+                    expected=len(matches),
+                    got=len(results),
+                    conversation_id=conversation_id,
+                )
+                return (
+                    "I apologize, sir. I was unable to complete the "
+                    "requested delegations."
+                )
+
             # Combine results (failures carry curated user-safe sentences)
             summaries = []
-            for (agent, task), item in zip(matches, results, strict=False):
+            for (agent, task), item in zip(matches, results, strict=True):
                 agent_name = agent.lower()
                 if isinstance(item, BaseException):
                     logger.error(