Feature/lab3 Tatyana Shmykova

[lime413]

Goal

Add a GitHub Actions PR gate for QuickNotes and document Lab 3 evidence, timings, and bonus optimization.

Changes

• Added CI for go vet, go test -race, and golangci-lint.
• Pinned runner, Go setup, checkout, and cache actions by full commit SHA.
• Added least-privilege workflow permissions.
• Added Go 1.23 and 1.24 matrix for vet and test.
• Added Go and linter caching plus path filters.
• Added branch protection and failed-run evidence to submissions/lab3.md.

Testing

• Ran go vet ./... locally.
• Ran go test -race -count=1 ./... locally.
• Verified latest GitHub Actions run is green:
  https://github.com/lime413/DevOps-Intro/actions/runs/27611706353
• Verified deliberate failing run and follow-up fix are documented.

Checklist

• Title is a clear sentence (≤ 70 chars)
• Commits are signed (git log --show-signature)
• submissions/labN.md updated