[feat] Add simple "labeled" merge-queue action

[luongvo]

What happened 👀

• Add the simple "labeled" merge queue action, alternative to GitHub's native merge queue. Inspired by opper-ai/pr-gatory-action, clone for customization & security purpose.

Insight 📝

What it does

Label a PR with mergeme and merge-queue takes over:

1. Rebases the PR onto the latest base branch
2. Waits for all status checks to pass on the rebased code
3. Squash merges into the base branch and deletes the source branch
4. Dequeues with a PR comment explaining what went wrong if anything fails

Multiple PRs labeled at the same time? They all get processed. When a job starts, it scans for every open PR with the label and works through them in order — so even if several PRs are labeled while one is already being processed, they'll all be picked up by the time the queue drains.

How the queue works

When triggered, the action scans for all open PRs carrying the label and processes them serially — oldest first. The concurrency block ensures only one job runs at a time, so a second trigger that arrives while the first job is busy will wait and then drain whatever remains in the queue when it starts.

PR #1 labeled  ─┐
PR #2 labeled  ─┤──>  job picks up #1, #2, #3 in order  ──>  all merged
PR #3 labeled  ─┘

Each PR rebases onto the base branch at the moment it's processed, which includes all previously merged PRs. This is the key property of a merge queue — no PR merges without being tested against the current state of the target branch.

What happens on failure

merge-queue removes the mergeme label and leaves a comment on the PR explaining what went wrong:

Failure	What merge-queue does
Merge conflicts	Removes from queue, comments to resolve conflicts
Rebase fails	Removes from queue, comments to rebase manually
Status checks fail	Removes from queue, comments to fix and re-label
Timeout (default 30 min)	Removes from queue, comments to re-label
Squash merge rejected	Removes from queue, comments about branch protection

To retry, fix the issue and add the mergeme label again.

Proof Of Work 📹

TBA