fix: preserve CLI session IDs for session resume

- Add resumeArgs to DEFAULT_CLAUDE_BACKEND for proper --resume flag usage
- Fix gateway not preserving cliSessionIds/claudeCliSessionId in nextEntry
- Add test for CLI session ID preservation in gateway agent handler
- Update docs with new resumeArgs default

src/gateway/server-methods/agent.ts

@@ -251,6 +251,8 @@ export const agentHandlers: GatewayRequestHandlers = {
         groupId: resolvedGroupId ?? entry?.groupId,
         groupChannel: resolvedGroupChannel ?? entry?.groupChannel,
         space: resolvedGroupSpace ?? entry?.space,
+        cliSessionIds: entry?.cliSessionIds,
+        claudeCliSessionId: entry?.claudeCliSessionId,
       };
       sessionEntry = nextEntry;
       const sendPolicy = resolveSendPolicy({