test: address Copilot review on push-queue helper and hook docs
Commit

- PushQueueRefs: resolve the git common dir via rev-parse like production (checkpoint/pushqueue.go) instead of hard-coding RepoDir/.git, so the helper is correct from linked worktrees
- clarify InstallRealPrePushHook/GitPushArgsWithHooks docs: the wrapper forwards only the remote name, matching the production hook template
Co-Authored-By: Claude Fable 5 noreply@anthropic.com Claude-Session: https://claude.ai/code/session_012yi3hHGAGepwfrfjPETjGq Entire-Checkpoint: 283495b6edfd
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