fix(ci): detach HEAD 释放本地分支名，避开 fork PR 同名冲突

[appergb]

User description

背景

PR #451 (fork aeoform/openless, head ref 也叫 beta) 上有人 @claude，云端 review action 失败：

failed run: https://github.com/Open-Less/openless/actions/runs/25978532215

错误链：

PR #451 is from a fork, fetching via refs/pull/451/head...
fatal: refusing to fetch into branch 'refs/heads/beta' checked out at '/home/runner/work/openless/openless'
##[error]Action failed with error: Command failed: git fetch origin --depth=20 pull/451/head:beta

根因（三连命中才会触发）

1. PR 来自 fork (isCrossRepository: true)
2. fork 的 head 分支名也叫 beta —— 与本仓库 base / 默认分支同名
3. actions/checkout@v4 默认把 worktree checkout 到了 refs/heads/beta

claude-code-action@v1 内部执行：

git fetch origin --depth=20 pull/451/head:beta

把 PR head 拉到本地同名分支 beta（命名取自 fork 端 head ref name）。但 beta 已是当前 checked-out 分支，git 拒绝 fetch 到已 checkout 的分支 → 失败。

修复

在 actions/checkout 之后立即 git checkout --detach，释放 refs/heads/<default-branch> 这个本地分支名，让 action 自己 fetch 不撞名。

仅对 fork head ref == base ref name 这个 corner case 有影响；其他场景（本仓库分支、fork 分支名不同）detach 都是无副作用的。

改动

• .github/workflows/claude.yml: +11 行，单 step + 注释，无其他改动。

Test plan

• 合入后在 PR feat(linux): fcitx5 plugin for Wayland input + deb packaging #451 上重新 @claude，确认 action run 成功，不再出现 refusing to fetch into branch 错误
• 在本仓库分支的 PR / Issue 上 @claude，确认正常路径不退化
• 若有第二个 fork PR head ref name 与 base 不同名，确认仍正常工作

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PR Type

Bug fix

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Description

• Detach checkout before Claude action

• Free default branch ref on forks

• Prevent fetch collisions in CI

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Diagram Walkthrough

flowchart LR
  A["actions/checkout"] -- "detach HEAD" --> B["Free local branch ref"]
  B -- "avoids ref collision" --> C["claude-code-action fetches PR head"]
  C -- "CI succeeds" --> D["Claude review workflow"]

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File Walkthrough

	Relevant files
Bug fix	claude.yml Detach HEAD before Claude review step                                        .github/workflows/claude.yml Adds a git checkout --detach step after repository checkout. Prevents claude-code-action from fetching into an already checked-out branch. Documents the fork PR ref-name collision scenario in workflow comments. +11/-0

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[github-actions]

PR Reviewer Guide 🔍

(Review updated until commit 00cffb3)

Here are some key observations to aid the review process:

⏱️ Estimated effort to review: 1 🔵⚪⚪⚪⚪

🧪 No relevant tests

🔒 No security concerns identified

⚡ No major issues detected

[github-actions]

Persistent review updated to latest commit 00cffb3