fix(explain): address review findings — N+1 queries, duplicate decisions, silent errors

1. fetch_open_threads: replace N+1 loop (2 queries per thread) with a
   single query using correlated subqueries for note_count and started_by.
2. extract_key_decisions: track consumed notes so the same note is not
   matched to multiple events, preventing duplicate decision entries.
3. build_timeline_excerpt_from_pipeline: log tracing::warn on seed/collect
   failures instead of silently returning empty timeline.

src/app/robot_docs.rs

@@ -460,7 +460,8 @@ fn handle_robot_docs(robot_mode: bool, brief: bool) -> Result<(), Box<dyn std::e
         "17": "Not found",
         "18": "Ambiguous match",
         "19": "Health check failed",
-        "20": "Config not found"
+        "20": "Config not found",
+        "21": "Embeddings not built"
     });
 
     let workflows = serde_json::json!({