feat: 前置驗證納入 git push 認證檢查

git push 走 askpass + HTTP 認證，與 Gitea REST API 是兩套機制，API token
有效不代表 push 能用（曾出現 askpass 無法執行、could not read Username 而
push 失敗）。新增 git.js verifyRemoteAccess() 以相同 askpass + remote URL
跑唯讀 git ls-remote，preflight 呼叫並在失敗時 exit 1，提前攔下設定問題。

新增 git.test.js 對 verifyRemoteAccess 的測試（成功、失敗不丟例外、token
不外洩、askpass 清理）。

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

app/git.test.js

@@ -3,7 +3,7 @@ import assert from 'node:assert/strict';
 import fs from 'fs';
 import os from 'os';
 import path from 'path';
-import { commitAndPush, cloneRepo, SYNC_PATHS, BOT_COMMIT_MARKER, getHeadCommitMessage, isBotAutoCommit, mergeInstructionText } from './git.js';
+import { commitAndPush, cloneRepo, verifyRemoteAccess, SYNC_PATHS, BOT_COMMIT_MARKER, getHeadCommitMessage, isBotAutoCommit, mergeInstructionText } from './git.js';
 
 // --- helpers ---
 function makeTmpWorkspace() {
@@ -334,3 +334,47 @@ describe('cloneRepo', () => {
     assert.equal(isBotAutoCommit(workspace, spawn), true);
   });
 });
+
+describe('verifyRemoteAccess', () => {
+  let workspace;
+  before(() => { workspace = fs.mkdtempSync(path.join(os.tmpdir(), 'git-lsremote-')); });
+  after(() => { fs.rmSync(workspace, { recursive: true, force: true }); });
+
+  it('runs git ls-remote with the askpass credential env and reports ok on success', () => {
+    const calls = [];
+    const spawn = (cmd, args, opts) => {
+      calls.push({ cmd, args, opts });
+      return { status: 0, stdout: 'abc123\tHEAD', stderr: '', error: null };
+    };
+    const result = verifyRemoteAccess(workspace, spawn);
+    assert.deepEqual(result, { ok: true });
+    const lsRemote = calls.find(c => c.args[0] === 'ls-remote');
+    assert.ok(lsRemote, 'expected git ls-remote to run');
+    assert.ok(lsRemote.opts?.env?.GIT_ASKPASS, 'expected GIT_ASKPASS env for ls-remote');
+  });
+
+  it('does not leak the token in ls-remote args', () => {
+    const calls = [];
+    const spawn = (cmd, args, opts) => {
+      calls.push({ args });
+      return { status: 0, stdout: '', stderr: '', error: null };
+    };
+    verifyRemoteAccess(workspace, spawn);
+    for (const { args } of calls) {
+      assert.ok(!args.join(' ').includes('test-token'), `Token leaked in git args: ${args.join(' ')}`);
+    }
+  });
+
+  it('reports failure (not throw) when git ls-remote fails', () => {
+    const spawn = () => ({ status: 128, stdout: '', stderr: 'fatal: could not read Username', error: null });
+    const result = verifyRemoteAccess(workspace, spawn);
+    assert.equal(result.ok, false);
+    assert.match(result.error, /could not read Username/);
+  });
+
+  it('cleans up the askpass script after running', () => {
+    verifyRemoteAccess(workspace, () => ({ status: 0, stdout: '', stderr: '', error: null }));
+    const leftover = fs.readdirSync(workspace).filter(f => f.endsWith('.git-askpass.sh'));
+    assert.equal(leftover.length, 0, 'askpass script was not cleaned up');
+  });
+});