fix(add-gmail): graceful startup when credentials missing + poll backoff

- connect() warns and returns instead of throwing when ~/.gmail-mcp/
  credentials are missing, preventing app crash
- index.ts wraps gmail.connect() in try/catch as a safety net
- Poll loop uses exponential backoff on consecutive errors (caps at 30m)
  instead of hammering the Gmail API every 60s on auth failures
- Switch from setInterval to setTimeout chain for proper backoff timing

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

.claude/skills/add-gmail/modify/src/index.ts

@@ -452,7 +452,11 @@ async function main(): Promise<void> {
 
   const gmail = new GmailChannel(channelOpts);
   channels.push(gmail);
-  await gmail.connect();
+  try {
+    await gmail.connect();
+  } catch (err) {
+    logger.warn({ err }, 'Gmail channel failed to connect, continuing without it');
+  }
 
   // Start subsystems (independently of connection handler)
   startSchedulerLoop({