ci: 引入分支保护配套工作流（path 过滤 → detect-changes，AI review 自动化） (#5)

## Summary / 概要

为了让 `main` 分支保护规则在所有 PR 上稳定生效，调整两个现有 i18n workflow 并新增两个 AI review 配套
workflow。

### 已配置的 main ruleset（ID 16298003）

- 只允许 squash merge，禁止 force push / 删除
- 必需 status check：`validate`、`translate`，require
`strict_required_status_checks_policy=true`（必须 rebase 最新）
- PR
rule：`required_approving_review_count=0`、`required_review_thread_resolution=true`、`dismiss_stale_reviews_on_push=true`
- `bypass_actors=[]`：管理员也不能绕过

### 本 PR 改动

1. **i18n-validate / i18n-translate**：移除 `on.pull_request.paths`，把过滤下移到
job 内 `detect-changes` step。路径不相关时跳过实际逻辑但 job 仍 success，避免 required
check 因 path 过滤缺失而卡死非 i18n PR。
2. **auto-request-copilot-review** (新增)：PR
opened/reopened/ready_for_review 时自动 `POST
/pulls/{n}/requested_reviewers` 加 Copilot 为 reviewer。
3. **wait-for-copilot-review** (新增)：PR
opened/reopened/synchronize/ready_for_review 时启动，轮询 reviews API 等待
`copilot-pull-request-reviewer[bot]` 在当前 head SHA 提交 review。超时 8 分钟则
fail。
- 选择 polling 而不是 `pull_request_review` 事件触发是因为：Copilot bot 触发的
`pull_request_review` 事件被 GitHub 视为 first-time contributor，workflow run
会卡在 `action_required` 状态需 maintainer 在 web UI 手动批准，无法自动化。

合并后会再 PATCH ruleset 16298003，把 `wait-for-copilot-review` 加入 required
status checks 作为强制门槛。

## Test plan

- [x] `i18n-validate` 在本 PR 上跑出 success（PR 仅触及
`.github/workflows/`，relevant=false，走 skip 分支）
- [x] `i18n-translate` 同上
- [x] `auto-request-copilot-review` 跑出 success，Copilot 被自动加为 reviewer
- [x] `wait-for-copilot-review` 在 polling 窗口内捕获到 Copilot review 并 pass
- [ ] 解决 Copilot 提的 conversations 后能 squash merge
- [ ] 合并后用 PATCH ruleset 把 `wait-for-copilot-review` 加入 required checks

.github/workflows/auto-request-copilot-review.yml

@@ -0,0 +1,39 @@
+name: Auto-request Copilot review
+
+on:
+  pull_request:
+    types: [opened, reopened, ready_for_review]
+
+permissions:
+  pull-requests: write
+
+jobs:
+  request:
+    if: github.event.pull_request.draft == false
+    runs-on: ubuntu-latest
+    timeout-minutes: 2
+    steps:
+      - name: Request Copilot reviewer
+        env:
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          PR_NUMBER: ${{ github.event.pull_request.number }}
+        run: |
+          set -e
+          if gh api "repos/${{ github.repository }}/pulls/$PR_NUMBER" \
+              --jq '.requested_reviewers[].login' \
+              | grep -qE '^(Copilot|copilot-pull-request-reviewer\[bot\])$'; then
+            echo "Copilot already requested as reviewer."
+            exit 0
+          fi
+          # 422 here usually means Copilot has already submitted a review (so the
+          # request slot is gone). Treat that as benign.
+          if ! resp=$(gh api -X POST \
+              "repos/${{ github.repository }}/pulls/$PR_NUMBER/requested_reviewers" \
+              -f 'reviewers[]=copilot-pull-request-reviewer[bot]' 2>&1); then
+            echo "$resp"
+            if echo "$resp" | grep -q '"status":"422"'; then
+              echo "POST returned 422; Copilot likely already reviewed."
+              exit 0
+            fi
+            exit 1
+          fi