fix(ci): 移除 Gitea workflow 外部 action 依赖

改为在 linux runner 中手动 fetch 当前提交，避免自托管 runner 准备阶段通过 gh-proxy 克隆 actions/checkout 和 dtolnay/rust-toolchain 时被 403 拦截，并同步开发文档与缺口说明。

.gitea/workflows/bat.yml

@@ -1,6 +1,7 @@
 # Gitea Actions workflow for the Rust workspace.
 # This workflow is linux-runner friendly and does not touch real resource
-# directories.
+# directories. It intentionally avoids external GitHub Actions so self-hosted
+# runners do not need to clone action repositories through a network proxy.
 
 name: bat-rust
 
@@ -21,21 +22,35 @@ jobs:
       BAT_SKIP_ENV_FILE: "1"
     steps:
       - name: Checkout
-        uses: actions/checkout@v4
+        shell: bash
+        run: |
+          set -euo pipefail
+          : "${GITHUB_SERVER_URL:?GITHUB_SERVER_URL is required}"
+          : "${GITHUB_REPOSITORY:?GITHUB_REPOSITORY is required}"
+          : "${GITHUB_SHA:?GITHUB_SHA is required}"
 
-      - name: Install Rust toolchain
-        uses: dtolnay/rust-toolchain@stable
-        with:
-          components: rustfmt,clippy
+          repo_url="${GITHUB_SERVER_URL%/}/${GITHUB_REPOSITORY}.git"
+          if [ -d .git ]; then
+            git remote set-url origin "${repo_url}" 2>/dev/null || git remote add origin "${repo_url}"
+          else
+            git init .
+            git remote add origin "${repo_url}"
+          fi
+
+          ref="${GITHUB_REF:-${GITHUB_SHA}}"
+          git fetch --no-tags --depth=1 origin "${ref}" || git fetch --no-tags --depth=1 origin "${GITHUB_SHA}"
+          git checkout --force --detach FETCH_HEAD
+          git submodule update --init --recursive
 
       - name: Show tool versions
         shell: bash
         run: |
           set -euo pipefail
+          command -v git
           rustc --version
           cargo --version
-          command -v curl
-          command -v unzip
+          rustfmt --version
+          cargo clippy --version
 
       - name: Check formatting
         shell: bash