feat: 階段一 - 改用 Node.js 實作基本流程骨架

- Dockerfile: 改用 node:20-slim
- entrypoint.sh: 執行 app/main.js
- app/package.json: axios + js-yaml + openai
- app/config.js: 環境變數與 LLM 自動偵測（10 種服務）
- app/llm.js: OpenAI-compatible 統一介面
- app/gitea.js: PR diff 取得與 comment 發布
- app/roles.js: 從 prompts/roles/*.yaml 載入角色
- app/main.js: pipeline 骨架，log 每個主要階段

app/gitea.js

@@ -0,0 +1,15 @@
+import axios from 'axios';
+import { GITEA_TOKEN, GITEA_SERVER_URL, GITEA_REPOSITORY, PR_NUMBER } from './config.js';
+
+const headers = () => ({ Authorization: `token ${GITEA_TOKEN}`, 'Content-Type': 'application/json' });
+const api = (path) => `${GITEA_SERVER_URL.replace(/\/$/, '')}/api/v1${path}`;
+
+export async function getPRDiff() {
+  const resp = await axios.get(api(`/repos/${GITEA_REPOSITORY}/pulls/${PR_NUMBER}.diff`), { headers: headers(), timeout: 60000 });
+  return resp.data;
+}
+
+export async function postComment(body) {
+  const resp = await axios.post(api(`/repos/${GITEA_REPOSITORY}/issues/${PR_NUMBER}/comments`), { body }, { headers: headers(), timeout: 30000 });
+  return resp.data;
+}