docs(lab3): submit Lab 3. add CI/CD pipeline with vet, test, lint

[linxel]

Goal

Add CI/CD pipeline with vet, test, lint as required PR gate for QuickNotes.

Changes

• Add .github/workflows/ci.yml with vet, test, lint jobs
• Pin runner to ubuntu-24.04
• Pin all third-party actions by full SHA with comments
• Set permissions: contents: read
• Enable cache for Go modules
• Add matrix for Go 1.23 and 1.24 with fail-fast: false
• Add path filter to skip docs-only changes
• Bonus: add Alpine image, GOFLAGS, lint on changed files

Testing

• CI runs and passes all checks on PR
• Deliberately broke test → CI became red (screenshot in submission)
• Fixed test → CI became green again
• Branch protection enabled requiring vet, test, lint

Checklist

• Title is a clear sentence (≤ 70 chars)
• Commits are signed (git log --show-signature)
• submissions/lab3.md updated