feat: OpenMesh 基础平台与 MD/PDF 转换技能

- 后端: coworker 智能体框架, WS API, 文件上传, 附件处理
- 前端: Open WebUI, 文件全量走 upload API (含 MD/TXT/JSON 等文本类)
- 技能: md-to-office (pandoc + wkhtmltopdf)
- 修复: 上传文件路径丢失, Agent 搜索浪费, 输出文件跑到 uploads/
- 打包: PyInstaller one-dir, 预打包 pandoc/wkhtmltopdf/chromium

surfaces/gui/src-tauri/capabilities/default.json

@@ -0,0 +1,15 @@
+{
+  "$schema": "../gen/schemas/desktop-schema.json",
+  "identifier": "default",
+  "description": "Capabilities for the main coworker window.",
+  "windows": ["main"],
+  "permissions": [
+    "core:default",
+    "core:window:allow-hide",
+    "core:window:allow-show",
+    "core:window:allow-set-focus",
+    "core:window:allow-unminimize",
+    "dialog:default",
+    "autostart:default"
+  ]
+}