fix(robot): propagate JSON serialization errors instead of silent failure

Three robot-mode print functions used `serde_json::to_string().unwrap_or_default()`
which silently outputs an empty string on failure (exit 0, no error). This
diverged from the codebase standard in handlers.rs which uses `?` propagation.

Changed to return Result<()> with proper LoreError::Other mapping:
- explain.rs: print_explain_json()
- file_history.rs: print_file_history_json()
- trace.rs: print_trace_json()

Updated callers in handlers.rs and explain.rs to propagate with `?`.

While serde_json::to_string on a json!() Value is unlikely to fail in practice
(only non-finite floats trigger it), the unwrap_or_default pattern violates the
robot mode contract: callers expect either valid JSON on stdout or a structured
error on stderr with a non-zero exit code, never empty output with exit 0.

src/app/handlers.rs

@@ -1328,7 +1328,7 @@ fn handle_file_history(
 
     if robot_mode {
         let elapsed_ms = start.elapsed().as_millis() as u64;
-        print_file_history_json(&result, elapsed_ms);
+        print_file_history_json(&result, elapsed_ms)?;
     } else {
         print_file_history(&result);
     }
@@ -1384,7 +1384,7 @@ fn handle_trace(
 
     if robot_mode {
         let elapsed_ms = start.elapsed().as_millis() as u64;
-        print_trace_json(&result, elapsed_ms, line_requested);
+        print_trace_json(&result, elapsed_ms, line_requested)?;
     } else {
         print_trace(&result);
     }

src/cli/commands/explain.rs

@@ -821,7 +821,8 @@ fn fetch_related_entities(
          LEFT JOIN merge_requests mr ON er.target_entity_type = 'merge_request' AND mr.id = er.target_entity_id \
          WHERE er.source_entity_type = ?1 AND er.source_entity_id = ?2 \
            AND er.reference_type != 'closes' \
-           AND er.target_entity_iid IS NOT NULL",
+           AND er.target_entity_iid IS NOT NULL \
+         ORDER BY er.target_entity_type, er.target_entity_iid",
     )?;
 
     let outgoing: Vec<RelatedEntityInfo> = out_stmt
@@ -845,7 +846,8 @@ fn fetch_related_entities(
          LEFT JOIN merge_requests mr ON er.source_entity_type = 'merge_request' AND mr.id = er.source_entity_id \
          WHERE er.target_entity_type = ?1 AND er.target_entity_id = ?2 \
            AND er.reference_type != 'closes' \
-           AND COALESCE(i.iid, mr.iid) IS NOT NULL",
+           AND COALESCE(i.iid, mr.iid) IS NOT NULL \
+         ORDER BY er.source_entity_type, COALESCE(i.iid, mr.iid)",
     )?;
 
     let incoming: Vec<RelatedEntityInfo> = in_stmt
@@ -1020,7 +1022,7 @@ pub fn handle_explain(
     let elapsed_ms = start.elapsed().as_millis() as u64;
 
     if robot_mode {
-        print_explain_json(&result, elapsed_ms);
+        print_explain_json(&result, elapsed_ms)?;
     } else {
         print_explain(&result);
     }
@@ -1032,13 +1034,18 @@ pub fn handle_explain(
 // Output rendering (Task 5 fills these in fully)
 // ---------------------------------------------------------------------------
 
-pub fn print_explain_json(result: &ExplainResult, elapsed_ms: u64) {
+pub fn print_explain_json(result: &ExplainResult, elapsed_ms: u64) -> Result<()> {
     let response = serde_json::json!({
         "ok": true,
         "data": result,
         "meta": { "elapsed_ms": elapsed_ms }
     });
-    println!("{}", serde_json::to_string(&response).unwrap_or_default());
+    println!(
+        "{}",
+        serde_json::to_string(&response)
+            .map_err(|e| LoreError::Other(format!("JSON serialization failed: {e}")))?
+    );
+    Ok(())
 }
 
 pub fn print_explain(result: &ExplainResult) {

src/cli/commands/file_history.rs

@@ -5,7 +5,7 @@ use tracing::info;
 use crate::Config;
 use crate::cli::render::{self, Icons, Theme};
 use crate::core::db::create_connection;
-use crate::core::error::Result;
+use crate::core::error::{LoreError, Result};
 use crate::core::file_history::resolve_rename_chain;
 use crate::core::paths::get_db_path;
 use crate::core::project::resolve_project;
@@ -391,7 +391,7 @@ pub fn print_file_history(result: &FileHistoryResult) {
 
 // ── Robot (JSON) output ─────────────────────────────────────────────────────
 
-pub fn print_file_history_json(result: &FileHistoryResult, elapsed_ms: u64) {
+pub fn print_file_history_json(result: &FileHistoryResult, elapsed_ms: u64) -> Result<()> {
     let output = serde_json::json!({
         "ok": true,
         "data": {
@@ -409,5 +409,10 @@ pub fn print_file_history_json(result: &FileHistoryResult, elapsed_ms: u64) {
         }
     });
 
-    println!("{}", serde_json::to_string(&output).unwrap_or_default());
+    println!(
+        "{}",
+        serde_json::to_string(&output)
+            .map_err(|e| LoreError::Other(format!("JSON serialization failed: {e}")))?
+    );
+    Ok(())
 }

src/cli/commands/trace.rs

@@ -1,4 +1,5 @@
 use crate::cli::render::{Icons, Theme};
+use crate::core::error::{LoreError, Result};
 use crate::core::trace::{TraceChain, TraceResult};
 
 /// Parse a path with optional `:line` suffix.
@@ -152,7 +153,11 @@ fn truncate_body(body: &str, max: usize) -> String {
     format!("{}...", &body[..boundary])
 }
 
-pub fn print_trace_json(result: &TraceResult, elapsed_ms: u64, line_requested: Option<u32>) {
+pub fn print_trace_json(
+    result: &TraceResult,
+    elapsed_ms: u64,
+    line_requested: Option<u32>,
+) -> Result<()> {
     // Truncate discussion bodies for token efficiency in robot mode
     let chains: Vec<serde_json::Value> = result
         .trace_chains
@@ -205,7 +210,12 @@ pub fn print_trace_json(result: &TraceResult, elapsed_ms: u64, line_requested: O
         }
     });
 
-    println!("{}", serde_json::to_string(&output).unwrap_or_default());
+    println!(
+        "{}",
+        serde_json::to_string(&output)
+            .map_err(|e| LoreError::Other(format!("JSON serialization failed: {e}")))?
+    );
+    Ok(())
 }
 
 #[cfg(test)]

src/ingestion/discussions_tests.rs

@@ -40,7 +40,11 @@ fn setup() -> Connection {
 }
 
 fn get_discussion_id(conn: &Connection) -> i64 {
-    conn.query_row("SELECT id FROM discussions LIMIT 1", [], |row| row.get(0))
+    conn.query_row(
+        "SELECT id FROM discussions ORDER BY id LIMIT 1",
+        [],
+        |row| row.get(0),
+    )
     .unwrap()
 }
 

src/ingestion/mr_discussions.rs

@@ -786,7 +786,11 @@ mod tests {
     }
 
     fn get_mr_discussion_id(conn: &Connection) -> i64 {
-        conn.query_row("SELECT id FROM discussions LIMIT 1", [], |row| row.get(0))
+        conn.query_row(
+            "SELECT id FROM discussions ORDER BY id LIMIT 1",
+            [],
+            |row| row.get(0),
+        )
         .unwrap()
     }
 

src/ingestion/storage/queue.rs

@@ -242,14 +242,16 @@ mod tests {
         .unwrap();
 
         let project_id: i64 = conn
-            .query_row("SELECT id FROM projects LIMIT 1", [], |row| row.get(0))
+            .query_row("SELECT id FROM projects ORDER BY id LIMIT 1", [], |row| {
+                row.get(0)
+            })
             .unwrap();
 
         enqueue_job(&conn, project_id, "issue", 42, 100, "resource_events", None).unwrap();
 
         let job_id: i64 = conn
             .query_row(
-                "SELECT id FROM pending_dependent_fetches LIMIT 1",
+                "SELECT id FROM pending_dependent_fetches ORDER BY id LIMIT 1",
                 [],
                 |row| row.get(0),
             )
@@ -301,7 +303,9 @@ mod tests {
         let (conn, _job_id) = setup_db_with_job();
 
         let project_id: i64 = conn
-            .query_row("SELECT id FROM projects LIMIT 1", [], |row| row.get(0))
+            .query_row("SELECT id FROM projects ORDER BY id LIMIT 1", [], |row| {
+                row.get(0)
+            })
             .unwrap();
         let jobs = claim_jobs(&conn, "resource_events", project_id, 10).unwrap();
         assert_eq!(jobs.len(), 1);