Lab 3: PR-gated CI for QuickNotes (vet + test + lint)

[danielpancake]

Goal

Add a PR-gated CI pipeline for QuickNotes that runs vet, test, and lint on every PR to main, hardened with SHA-pinned actions and sped up with caching and a Go version matrix.

Changes

• .github/workflows/ci.yml: three independent required units (vet, test, lint) on pinned ubuntu-24.04, Go pinned, golangci-lint v2.5.0 pinned.
• Security: every third-party action pinned by 40-char commit SHA (with tag comment); permissions: contents: read at workflow level.
• Speed (Task 2): Go module/build cache, 1.23 + 1.24 matrix on vet/test (fail-fast: false), path filter (app/**, .github/workflows/**), and a single ci-ok aggregation gate.
• submissions/lab3.md: chosen path, evidence screenshots, baseline/cache/cache+matrix timing table, and design answers a-h.

Testing

• Green run with all required checks passing: https://github.com/danielpancake/DevOps-Intro/actions/runs/27645771948
• Gate proven to block: broke a test (test red, merge blocked) in 0f250b6, reverted to green in 19c587c.
• Branch protection on the fork's main requires the ci-ok check (strict, admin-enforced).
• Path filter verified: a submission-only push skipped CI (no run created).

Checklist

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