feat(agent): add maxConcurrent config for parallel message handling

Adds `agent.maxConcurrent` config option to control how many agent runs
can execute in parallel across all conversations. Default remains 1
(sequential) for backwards compatibility.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/gateway/server.ts

@@ -1736,6 +1736,7 @@ export async function startGatewayServer(
     { controller: AbortController; sessionId: string; sessionKey: string }
   >();
   setCommandLaneConcurrency("cron", cfgAtStart.cron?.maxConcurrentRuns ?? 1);
+  setCommandLaneConcurrency("main", cfgAtStart.agent?.maxConcurrent ?? 1);
 
   const cronStorePath = resolveCronStorePath(cfgAtStart.cron?.store);
   const cronLogger = getChildLogger({