feat: per-group queue, SQLite state, graceful shutdown

Add per-group container locking with global concurrency limit to prevent
concurrent containers for the same group (#89) and cap total containers.
Fix message batching bug where lastAgentTimestamp advanced to trigger
message instead of latest in batch, causing redundant re-processing.
Move router state, sessions, and registered groups from JSON files to
SQLite with automatic one-time migration. Add SIGTERM/SIGINT handlers
with graceful shutdown (SIGTERM -> grace period -> SIGKILL). Add startup
recovery for messages missed during crash. Remove dead code: utils.ts,
Session type, isScheduledTask flag, ContainerConfig.env, getTaskRunLogs,
GroupQueue.isActive.

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

src/config.ts

@@ -31,6 +31,10 @@ export const CONTAINER_MAX_OUTPUT_SIZE = parseInt(
   10,
 ); // 10MB default
 export const IPC_POLL_INTERVAL = 1000;
+export const MAX_CONCURRENT_CONTAINERS = parseInt(
+  process.env.MAX_CONCURRENT_CONTAINERS || '3',
+  10,
+);
 
 function escapeRegex(str: string): string {
   return str.replace(/[.*+?^${}()|[\]\\]/g, '\\$&');