chore: collapse CI into single check job

[maxence2997]

Summary

Simplify the CI workflow so the real verification job is the required check instead of running a second no-op gate job. This keeps the same required check name while avoiding an extra runner allocation.

Changes

• renamed the real lint-test job to check
• removed the second gate-only check job that only depended on lint-test
• kept the actual fmt, lint, and test steps unchanged

Checklist

Required

• make check passes (fmt → lint → test)
• Each commit represents exactly one logical change
• Commit messages follow the format in commit-message-instructions.md
• No unrelated code reformatting in this PR

Conditional (list only those that apply)

• Performance-sensitive change: not applicable

[Copilot]

Pull request overview

Simplifies the GitHub Actions CI workflow by making the real verification job the required check job, eliminating the separate gate-only job while preserving the required check name.

Changes:

• Renamed the existing lint-test job to check (including the displayed job name).
• Removed the gate-only check job that depended on lint-test.
• Kept the existing fmt, lint, and test steps unchanged.