refactor(cleanup-release): 移除 RUNNER_TOKEN 輸入，一律改用 Gitea 自動 token

- action.yml 移除 RUNNER_TOKEN input 與 secrets 回退鏈，env 簡化為 GITEA_TOKEN: ${{ gitea.token }}
- src/index.js 環境變數與 log 訊息同步改名為 GITEA_TOKEN，遮罩行為不變
- 自動 token 權限僅限當前 repo 且 job 結束即失效，已足夠本 action 的 release/tag 清理需求

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/index.js

@@ -400,7 +400,7 @@ function hasBatchFailure(results) {
 async function main() {
   const GITEA_SERVER_URL = normalizeEnvValue(process.env.GITEA_SERVER_URL);
   const GITEA_REPOSITORY = normalizeEnvValue(process.env.GITEA_REPOSITORY);
-  const RUNNER_TOKEN = normalizeEnvValue(process.env.RUNNER_TOKEN) ?? '';
+  const GITEA_TOKEN = normalizeEnvValue(process.env.GITEA_TOKEN) ?? '';
   const KEEP_COUNT = normalizeEnvValue(process.env.KEEP_COUNT) ?? '';
   const MAX_PAGES = normalizeEnvValue(process.env.MAX_PAGES) ?? '';
 
@@ -421,11 +421,11 @@ async function main() {
 
   const keepCount = Number(KEEP_COUNT);
   const authHeaders = {};
-  if (isEmptyOrNull(RUNNER_TOKEN)) {
-    warn('RUNNER_TOKEN is empty; release API calls will be anonymous');
+  if (isEmptyOrNull(GITEA_TOKEN)) {
+    warn('GITEA_TOKEN is empty; release API calls will be anonymous');
   } else {
-    info('RUNNER_TOKEN=[redacted]');
-    authHeaders.Authorization = `token ${RUNNER_TOKEN}`;
+    info('GITEA_TOKEN=[redacted]');
+    authHeaders.Authorization = `token ${GITEA_TOKEN}`;
   }
 
   const serverBase = new URL(GITEA_SERVER_URL);