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!(

src/app/handlers.rs

@@ -735,7 +735,7 @@ async fn handle_init(
         }
 
         let project_paths: Vec<String> = projects_flag
-            .unwrap()
+            .expect("validated: checked for None at lines 714-721")
             .split(',')
             .map(|p| p.trim().to_string())
             .filter(|p| !p.is_empty())
@@ -743,8 +743,10 @@ async fn handle_init(
 
         let result = run_init(
             InitInputs {
-                gitlab_url: gitlab_url_flag.unwrap(),
-                token_env_var: token_env_var_flag.unwrap(),
+                gitlab_url: gitlab_url_flag
+                    .expect("validated: checked for None at lines 714-721"),
+                token_env_var: token_env_var_flag
+                    .expect("validated: checked for None at lines 714-721"),
                 project_paths,
                 default_project: default_project_flag.clone(),
             },

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!({