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/utils.ts

@@ -1,18 +0,0 @@
-import fs from 'fs';
-import path from 'path';
-
-export function loadJson<T>(filePath: string, defaultValue: T): T {
-  try {
-    if (fs.existsSync(filePath)) {
-      return JSON.parse(fs.readFileSync(filePath, 'utf-8'));
-    }
-  } catch {
-    // Return default on error
-  }
-  return defaultValue;
-}
-
-export function saveJson(filePath: string, data: unknown): void {
-  fs.mkdirSync(path.dirname(filePath), { recursive: true });
-  fs.writeFileSync(filePath, JSON.stringify(data, null, 2));
-}