fix(robot): replace JSON serialization unwrap with graceful error handling

Replace serde_json::to_string(&output).unwrap() with match-based error
handling across all robot-mode JSON printers. On serialization failure,
the error is now written to stderr instead of panicking. This hardens
the CLI against unexpected Serialize failures in production.

Affected commands: count (2), embed, generate-docs, ingest (2), search,
stats, sync (2), sync-status, timeline.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/cli/commands/sync.rs

@@ -746,7 +746,10 @@ pub fn print_sync_json(result: &SyncResult, elapsed_ms: u64, metrics: Option<&Me
             stages,
         },
     };
-    println!("{}", serde_json::to_string(&output).unwrap());
+    match serde_json::to_string(&output) {
+        Ok(json) => println!("{json}"),
+        Err(e) => eprintln!("Error serializing to JSON: {e}"),
+    }
 }
 
 #[derive(Debug, Default, Serialize)]
@@ -880,7 +883,10 @@ pub fn print_sync_dry_run_json(result: &SyncDryRunResult) {
         },
     };
 
-    println!("{}", serde_json::to_string(&output).unwrap());
+    match serde_json::to_string(&output) {
+        Ok(json) => println!("{json}"),
+        Err(e) => eprintln!("Error serializing to JSON: {e}"),
+    }
 }
 
 #[cfg(test)]