fix: release session locks on process termination

Adds cleanup handlers to release held file locks when the process
terminates via SIGTERM, SIGINT, or normal exit. This prevents orphaned
lock files that would block future sessions.

Fixes #1951

src/agents/session-write-lock.ts

@@ -14,6 +14,32 @@ type HeldLock = {
 
 const HELD_LOCKS = new Map<string, HeldLock>();
 
+/**
+ * Release all held locks - called on process exit to prevent orphaned locks
+ */
+async function releaseAllLocks(): Promise<void> {
+  const locks = Array.from(HELD_LOCKS.values());
+  HELD_LOCKS.clear();
+  for (const lock of locks) {
+    try {
+      await lock.handle.close();
+      await fs.rm(lock.lockPath, { force: true });
+    } catch {
+      // Best effort cleanup
+    }
+  }
+}
+
+// Register cleanup handlers to release locks on unexpected termination
+process.on("exit", releaseAllLocks);
+process.on("SIGTERM", () => {
+  void releaseAllLocks().then(() => process.exit(0));
+});
+process.on("SIGINT", () => {
+  void releaseAllLocks().then(() => process.exit(0));
+});
+// Note: unhandledRejection handler will call process.exit() which triggers 'exit'
+
 function isAlive(pid: number): boolean {
   if (!Number.isFinite(pid) || pid <= 0) return false;
   try {