feat(mqtt): finalize dashboard screenshot payload v2 and trigger flow

- switch dashboard payload to grouped schema v2.0 in simclient
- support immediate event-triggered screenshot sends via meta.json signaling
- update README and copilot instructions to document v2 payload and trigger behavior
- update migration checklist to reflect completed client/server rollout

src/simclient.py

@@ -698,19 +698,6 @@ def _build_dashboard_payload(client_id: str, screenshot_info: dict, health: dict
         }
 
     payload = {
-        # Legacy fields kept during migration so existing server parsing remains intact.
-        "timestamp": published_at,
-        "client_id": client_id,
-        "status": "alive",
-        "screenshot_type": capture_type,
-        "screenshot": screenshot_info,
-        "screenshot_age_s": screenshot_age_s,
-        "system_info": {
-            "hostname": socket.gethostname(),
-            "ip": get_ip(),
-            "uptime": time.time()  # Could be replaced with actual uptime
-        },
-        # New grouped schema (v2-compat)
         "message": {
             "client_id": client_id,
             "status": "alive",
@@ -727,7 +714,7 @@ def _build_dashboard_payload(client_id: str, screenshot_info: dict, health: dict
             "process_health": process_health_payload,
         },
         "metadata": {
-            "schema_version": "2.0-compat",
+            "schema_version": "2.0",
             "producer": "simclient",
             "published_at": published_at,
             "capture": capture_meta,
@@ -739,9 +726,6 @@ def _build_dashboard_payload(client_id: str, screenshot_info: dict, health: dict
         },
     }
 
-    if process_health_payload:
-        payload["process_health"] = process_health_payload
-
     return payload
 
 
@@ -766,10 +750,14 @@ def send_screenshot_heartbeat(client, client_id, capture_type: str = "periodic",
         payload = json.dumps(heartbeat_data)
         res = client.publish(dashboard_topic, payload, qos=0)
         if res.rc == mqtt.MQTT_ERR_SUCCESS:
+            age_str = f", age={heartbeat_data['metadata']['capture']['age_s']}s" if heartbeat_data['metadata']['capture']['age_s'] is not None else ""
             if screenshot_info:
-                logging.info(f"Dashboard heartbeat sent with screenshot: {screenshot_info['filename']} ({screenshot_info['size']} bytes)")
+                logging.info(
+                    f"Dashboard published: schema=2.0 type={capture_type}"
+                    f" screenshot={screenshot_info['filename']} ({screenshot_info['size']} bytes){age_str}"
+                )
             else:
-                logging.info("Dashboard heartbeat sent (no screenshot available)")
+                logging.info(f"Dashboard published: schema=2.0 type={capture_type} (no screenshot)")
         elif res.rc == mqtt.MQTT_ERR_NO_CONN:
             logging.warning("Dashboard heartbeat publish returned NO_CONN; will retry on next interval")
         else: