refactor: extract dashboard and message-runtime from index.ts

- Extract dashboard code into src/dashboard.ts
- Extract message runtime into src/message-runtime.ts
- Improve group-queue with better concurrency handling
- Update agent-runner with enhanced env/config passing
- Simplify DB operations and cleanup unused code
- Update README for Codex SDK architecture

src/db.test.ts

@@ -3,12 +3,15 @@ import { describe, it, expect, beforeEach } from 'vitest';
 import {
   _initTestDatabase,
   createTask,
+  deleteSession,
   deleteTask,
   getAllChats,
   getAllRegisteredGroups,
   getMessagesSince,
   getNewMessages,
+  getSession,
   getTaskById,
+  setSession,
   setRegisteredGroup,
   storeChatMetadata,
   storeMessage,
@@ -300,6 +303,16 @@ describe('getNewMessages', () => {
   });
 });
 
+describe('session accessors', () => {
+  it('deletes only the current service session for a group', () => {
+    setSession('group-a', 'session-123');
+    expect(getSession('group-a')).toBe('session-123');
+
+    deleteSession('group-a');
+    expect(getSession('group-a')).toBeUndefined();
+  });
+});
+
 // --- storeChatMetadata ---
 
 describe('storeChatMetadata', () => {