fix: 消除死代码 — context-manager 接入 + mcp-client 接线 + 删除 document-processor

- agent-engine.ts: 接入 context-manager buildContext()，长对话自动滑动窗口+摘要压缩+token 裁剪
- tool-registry.ts: 导入 mcp-client，getEnabledToolDefinitions() 合并 MCP 工具缓存
- main.ts: 启动时 refreshMCPTools() 注册已启用的 MCP 工具
- 删除 document-processor.ts（RAG 遗留，完全无引用）

src/renderer/main.ts

@@ -24,7 +24,7 @@ import { initToolsModal, closeToolsModal } from './components/tools-modal.js';
 import { initMemoryManager } from './services/memory-manager.js';
 import { restoreHeartbeat } from './components/settings-modal.js';
 import { restoreCronTasks } from './services/cron-manager.js';
-import { setToolEnabled, setRunCommandMode } from './services/tool-registry.js';
+import { setToolEnabled, setRunCommandMode, refreshMCPTools } from './services/tool-registry.js';
 import { initToolConfirmModal } from './components/tool-confirm-modal.js';
 import { initWorkspacePanel } from './components/workspace-panel.js';
 import { initLogPanel, addLog } from './services/log-service.js';
@@ -361,6 +361,9 @@ async function init(): Promise<void> {
     const toggleTC = document.querySelector<HTMLInputElement>('#toggleToolCalling');
     if (toggleTC) toggleTC.checked = toolCallingEnabled;
 
+    // ── MCP 工具注册 ──
+    await refreshMCPTools();
+
     // ── Agent 记忆设置 ──
     const memoryEnabled = await db.getSetting('memoryEnabled', true);
     state.set('memoryEnabled', memoryEnabled);