ci(lab3): PR gate, cache, matrix, path filter & bonus opts

[markovav-official]

Goal

Add GitHub Actions CI for QuickNotes (vet, test, lint) with Task 2 optimizations and bonus performance work, documented in submissions/lab3.md.

Changes

• .github/workflows/ci.yml — PR gate (vet, test -race, golangci-lint v2.5.0), SHA-pinned actions, permissions: contents: read
• Task 2 — Go cache, matrix Go 1.23/1.24, path filter (app/**, .github/workflows/**), ci-ok aggregation job
• Bonus — GOFLAGS=-buildvcs=false, shallow clone, golangci-lint cache, skip lint on non-Go diffs (dorny/paths-filter)
• submissions/lab3.md — design answers, timing tables, failed/green run evidence, branch protection screenshots

Testing

• Green CI: https://github.com/markovav-official/DevOps-Intro/actions/runs/27529469084
• Deliberate failure blocked: https://github.com/markovav-official/DevOps-Intro/actions/runs/27528969661
• Fixed and green: https://github.com/markovav-official/DevOps-Intro/actions/runs/27529130257
• Bonus opt 4 (lint skipped): https://github.com/markovav-official/DevOps-Intro/actions/runs/27531535180
• Branch protection requires ci-ok on main

Checklist

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