fix: guard json.loads() against invalid TTS and skill_view responses

Two code paths call json.loads() on output from external tools without
catching JSONDecodeError. If the tool returns a non-JSON string (error
message, empty string, or None), the entire call path crashes.

1. gateway/run.py — text_to_speech_tool() result in voice reply path.
   A TTS failure that returns an error string instead of JSON crashes
   the voice reply handler, killing the message response entirely.

2. cron/scheduler.py — skill_view() result when loading skills for
   cron jobs. A corrupted or missing skill file that returns an error
   string instead of JSON crashes the cron tick, preventing all jobs
   from executing that cycle.

Both fixes catch (json.JSONDecodeError, TypeError), log a warning,
and gracefully skip the failed operation instead of crashing.

gateway/run.py

@@ -10592,7 +10592,11 @@ class GatewayRunner:
             result_json = await asyncio.to_thread(
                 text_to_speech_tool, text=tts_text, output_path=audio_path
             )
-            result = json.loads(result_json)
+            try:
+                result = json.loads(result_json)
+            except (json.JSONDecodeError, TypeError):
+                logger.warning("Auto voice reply TTS returned invalid JSON: %s", result_json[:200] if result_json else result_json)
+                return
 
             # Use the actual file path from result (may differ after opus conversion)
             actual_path = result.get("file_path", audio_path)