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/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(