feat(gateway): per-platform gateway_restart_notification flag

Adds an opt-out toggle on PlatformConfig that gates both restart
lifecycle pings: the "♻ Gateway restarted" message sent to the chat
that issued /restart, and the "♻️ Gateway online" home-channel
startup notification. Defaults to True so existing deployments are
unaffected.

The motivating split is operator vs. end-user surfaces: a back-channel
like Telegram should keep these pings, while a Slack workspace shared
with end users should not surface gateway lifecycle noise.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

gateway/run.py

@@ -11386,6 +11386,14 @@ class GatewayRunner:
                 )
                 return None
 
+            platform_cfg = self.config.platforms.get(platform)
+            if platform_cfg is not None and not platform_cfg.gateway_restart_notification:
+                logger.info(
+                    "Restart notification suppressed: %s has gateway_restart_notification=false",
+                    platform_str,
+                )
+                return None
+
             metadata = {"thread_id": thread_id} if thread_id else None
             result = await adapter.send(
                 str(chat_id),
@@ -11437,6 +11445,14 @@ class GatewayRunner:
             if not home or not home.chat_id:
                 continue
 
+            platform_cfg = self.config.platforms.get(platform)
+            if platform_cfg is not None and not platform_cfg.gateway_restart_notification:
+                logger.info(
+                    "Home-channel startup notification suppressed: %s has gateway_restart_notification=false",
+                    platform.value,
+                )
+                continue
+
             target = (platform.value, str(home.chat_id), str(home.thread_id) if home.thread_id else None)
             if target in skipped or target in delivered:
                 continue