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/config/config.ts

@@ -325,6 +325,8 @@ export type ClawdisConfig = {
     typingIntervalSeconds?: number;
     /** Periodic background heartbeat runs (minutes). 0 disables. */
     heartbeatMinutes?: number;
+    /** Max concurrent agent runs across all conversations. Default: 1 (sequential). */
+    maxConcurrent?: number;
     /** Bash tool defaults. */
     bash?: {
       /** Default time (ms) before a bash command auto-backgrounds. */
@@ -582,6 +584,7 @@ const ClawdisSchema = z.object({
       mediaMaxMb: z.number().positive().optional(),
       typingIntervalSeconds: z.number().int().positive().optional(),
       heartbeatMinutes: z.number().nonnegative().optional(),
+      maxConcurrent: z.number().int().positive().optional(),
       bash: z
         .object({
           backgroundMs: z.number().int().positive().optional(),

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({