fix: checkout merge commit for merged PRs in auto-tag workflow

[hudeng-go]

问题

当 PR 来自 fork 且合并后 fork 分支被删除时，auto-tag workflow 的 actions/checkout@v7 步骤会失败，因为：

1. PR 被合并后，workflow 由 pull_request_target: closed 触发
2. ref: refs/heads/${{ github.event.pull_request.head.ref }} 指向 fork 中的分支
3. fork 的分支在 merge 后被删除，checkout 无法找到该 ref
4. repository: ${{ github.event.pull_request.head.repo.full_name }} 指向 fork 而非 base repo

修复

在 actions/checkout@v7 中增加条件判断：

• PR 已合并（merged=true）：从 base repo（github.event.repository.full_name）检出 merge_commit_sha，该 commit 永久存在于 base repo 中
• PR 未合并（opened/synchronize）：保持原有行为，从 fork 检出分支 ref

变更文件

文件	修改处数
.github/workflows/auto-tag.yml	1

相关 Issue

• chore: bump version to 0.9.0 treeland#1312 auto-tag workflow checkout failure after merge (branch deleted from fork)

Summary by Sourcery

Bug Fixes:

• Ensure the auto-tag workflow can check out merged pull requests even when the source fork branch has been deleted.

[sourcery-ai]

Reviewer's guide (collapsed on small PRs)

Reviewer's Guide

Updates the auto-tag GitHub Actions workflow to correctly checkout merged pull requests from the base repository’s merge commit when the PR branch in a fork has been deleted, while preserving existing behavior for unmerged PRs.

File-Level Changes

Change	Details	Files
Adjust auto-tag workflow checkout logic to handle merged PRs from forks whose branches may be deleted.	Replace the static head ref with a conditional ref that uses the PR merge commit SHA when the PR is merged, otherwise falls back to the original head ref format. Change the repository selection to use the base repository for merged PRs while continuing to use the PR head repository for unmerged PRs. Keep existing checkout safety options (allow-unsafe-pr-checkout, persist-credentials) unchanged to maintain security behavior.	.github/workflows/auto-tag.yml

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[sourcery-ai]

Hey - I've left some high level feedback:

• Consider adding a fallback for cases where github.event.pull_request.merge_commit_sha is null or empty (e.g. squash/rebase merges), so the ref expression doesn’t resolve to an invalid value and cause checkout failures (for example falling back to github.sha).
• You could tighten the security posture by only setting allow-unsafe-pr-checkout: true when actually checking out from the fork (non-merged case), and disabling it when checking out the merge commit from the base repository.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- Consider adding a fallback for cases where `github.event.pull_request.merge_commit_sha` is null or empty (e.g. squash/rebase merges), so the `ref` expression doesn’t resolve to an invalid value and cause checkout failures (for example falling back to `github.sha`).
- You could tighten the security posture by only setting `allow-unsafe-pr-checkout: true` when actually checking out from the fork (non-merged case), and disabling it when checking out the merge commit from the base repository.

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[deepin-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: hudeng-go, wineee

The full list of commands accepted by this bot can be found here.

Details

Needs approval from an approver in each of these files:

• debian/deepin/OWNERS

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