Add lefthook hooks for commit message linting

What does this MR do and why?

Add commit-msg and pre-push lefthook hooks that lint commit messages using the Gitlab::Dangerfiles::CommitLinter class from the gitlab-dangerfiles gem — the same linter that Dangerbot runs in CI.

Currently, the commit message linting rules are only enforced by Dangerbot at CI time (via the danger-review job). This means developers don't get feedback until the MR pipeline runs. This MR adds local lefthook checks to provide earlier feedback:

• commit-msg hook: Immediate feedback when creating a commit.
• pre-push command: Blocks pushes containing commits with linting errors, preventing them from reaching the MR.

Both hooks share a single script (scripts/lint/lint_commits.rb) that uses CommitLinter directly, enforcing the exact same rules as Dangerbot (72-char line length, minimum 3 words, capitalization, no trailing period, no emoji, no short references, etc.) with zero duplicated logic.

The script auto-detects its mode based on arguments:

• With a file path (commit-msg): lints the message from the file.
• Without arguments (pre-push): lints all commits from merge-base to HEAD.

Both modes skip auto-generated commits (fixup!, squash!, merge, revert, suggestion).

References

• Discussed in Slack about making commit message linting a blocking lefthook check (Dangerbot would still catch commits from Web IDE)
• gitlab-dangerfiles CommitLinter — the linter class used by the new script
• commit_messages Danger rule — the CI-side rule we're replicating locally
• Commit messages guidelines

Screenshots or screen recordings

Not applicable — no UI changes.

How to set up and validate locally

1. Ensure lefthook is installed (lefthook install).
2. Try creating a commit with a subject longer than 72 characters:

   git commit --allow-empty -m 'This is a very long commit message subject line that definitely exceeds seventy-two characters'

   The commit-msg hook should reject it with an error.
3. Try creating a commit with a long body line:

   git commit --allow-empty -m 'Short subject line here' -m 'This is a very long body line that definitely exceeds seventy-two characters in the commit message body'

   The commit-msg hook should reject it with an error.
4. Verify that fixup! commits are allowed regardless of length:

   git commit --allow-empty -m 'fixup! This is a very long fixup commit message subject line that definitely exceeds seventy-two characters'

   The hook should allow it.
5. Verify that URL-only body lines are allowed:

   git commit --allow-empty -m 'A valid subject line' -m 'https://docs.gitlab.com/development/contributing/merge_request_workflow/#commit-messages-guidelines-which-is-a-very-long-url'

   The hook should allow it.

MR acceptance checklist

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