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);
     }