feat(check): add --no-merge option to reject merge commits

[gcornut]

By default, merge commits are silently skipped. This adds a --no-merge option to fail on merge commits instead, useful for enforcing a rebase workflow in CI.

Let me know if there any cases where adding this on our PR CI would be a bad idea.

Changes

• check_message.sh: --no-merge flag (or COMMIT_VALIDATOR_NO_MERGE env var) makes merge commits fail instead of being skipped, both for MERGE_MSG path and message-based detection.
• check.sh: when COMMIT_VALIDATOR_NO_MERGE is set, merge commits are included in the validated range and rejected.
• action.yml: new no_merge input wired to COMMIT_VALIDATOR_NO_MERGE.
• Tests: 5 new bats tests covering both scripts.
• README: documented the new option in both the GitHub Action and pre-commit hook sections.

Note on GitHub Actions pull_request workflows

When a pull_request workflow runs, HEAD is not the PR's head commit — GitHub creates a synthetic merge commit (PR branch merged into base) that is never pushed to the repository. With no_merge: "true", this could cause a false positive if the validated range included HEAD.

This is safe because the action already passes an explicit range using base.sha..head.sha, so the synthetic merge commit at HEAD is never part of the validated range.

[gcornut]

Tested here https://github.com/lumapps/lumapps-web/pull/16897

[gcornut]

We will actually have a GH repo config that does that better
https://github.com/lumapps/github-automation/pull/2525