Feature/lab3

[Dnau15]

Goal

Add a PR-gated CI pipeline that blocks merges to main unless vet, test, and lint pass.

Changes

• Added .github/workflows/ci.yml (push to main + PRs to main).
• Three jobs against app/: go vet, go test -race, golangci-lint (v2.5.0).
• Pinned ubuntu-24.04, actions by SHA, permissions: contents: read.
• Go module cache, matrix (1.23/1.24, fail-fast: false), ci-ok aggregation check, docs-only path filter.
• Enabled branch protection on main; added submissions/lab3.md.

Testing

• Iterated PR to green; broke a test to confirm the gate blocks merge, then reverted to green.
• Verified docs-only PR skips CI; recorded baseline/cached/matrix timings.

Checklist

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