ci(lab3): add PR-gated pipeline with vet, test, and lint

[blacktree-lab]

Goal:

Add CI pipeline with vet, test, and lint as a required PR gate for QuickNotes.

Changes:

• Added .github/workflows/ci.yml with vet, test, lint jobs
• Added Go build matrix (1.23 + 1.24)
• Added module caching, shallow clone, and path filter
• Added submissions/lab3.md with design question answers and evidence

Testing:

• CI runs green on all 5 matrix jobs (vet×2, test×2, lint)
• Deliberately broke test — PR was blocked as expected
• Fixed and confirmed green again
• Path filter verified — README-only commit skipped CI
• Branch protection requires all 3 checks to pass before merge

Checklist:

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