ci(lab3): add PR-gated workflow

[tivdzualubem]

Goal

Add the initial Lab 3 GitHub Actions PR gate.

Changes

• Added GitHub Actions CI workflow
• Added vet, test, and lint jobs
• Added Go cache, Go 1.23/1.24 matrix, path filters, pinned runner, pinned actions, and least-privilege permissions

Testing

• YAML parsed locally
• go vet ./... passed locally
• go test -race -count=1 ./... passed locally

Checklist

• Title is a clear sentence
• Commits are signed
• CI workflow added