Feature/lab3

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Goal

Submit Lab 3 — CI/CD: A PR-Gated Pipeline for QuickNotes

Changes

• Added .github/workflows/ci.yml with three independent jobs (vet, test, lint), runner pinned to ubuntu-24.04 and all actions pinned by full SHA, plus permissions: contents: read.
• Enabled dependency caching via setup-go (cache-dependency-path: app/go.sum).
• Added a Go 1.23/1.24 matrix on vet and test with fail-fast: false.
• Scoped the pull_request trigger with a paths: filter so docs-only PRs skip CI.

Testing

• Pushed the pipeline and confirmed a green run on all jobs.
• Deliberately broke a test to prove the gate blocks merge, then reverted to green.
• Recorded wall-clock for baseline / cache / cache+matrix.

Checklist

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