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/errors.rs

@@ -7,6 +7,10 @@ struct FallbackErrorOutput {
 struct FallbackError {
     code: String,
     message: String,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    suggestion: Option<String>,
+    #[serde(skip_serializing_if = "Vec::is_empty")]
+    actions: Vec<String>,
 }
 
 fn handle_error(e: Box<dyn std::error::Error>, robot_mode: bool) -> ! {
@@ -20,6 +24,8 @@ fn handle_error(e: Box<dyn std::error::Error>, robot_mode: bool) -> ! {
                         error: FallbackError {
                             code: "INTERNAL_ERROR".to_string(),
                             message: gi_error.to_string(),
+                            suggestion: None,
+                            actions: Vec::new(),
                         },
                     };
                     serde_json::to_string(&fallback)
@@ -59,6 +65,8 @@ fn handle_error(e: Box<dyn std::error::Error>, robot_mode: bool) -> ! {
             error: FallbackError {
                 code: "INTERNAL_ERROR".to_string(),
                 message: e.to_string(),
+                suggestion: None,
+                actions: Vec::new(),
             },
         };
         eprintln!(