ci(lab3): add PR-gated pipeline with caching, matrix, optimizations

[infernaltiger]

Goal

Implement a PR-gated CI pipeline for QuickNotes with caching, build matrix, path filtering, and performance optimizations.

Changes

• Added .github/workflows/ci.yml with three independent jobs: vet, test, lint
• Set permissions: contents: read (least privilege principle)
• Pinned runner to ubuntu-24.04 and golangci-lint to v2.5.0
• Enabled Go module caching via actions/setup-go keyed by app/go.sum
• Added build matrix for Go 1.23 and 1.24 with fail-fast: false
• Added path filtering to skip pipeline on docs-only changes
• Added bonus optimizations: manual golangci-lint cache, concurrency group, checkout tuning
• Configured branch protection on fork's main requiring CI to pass
• Added submissions/lab3.md with timing tables, design question answers, and bottleneck analysis

Testing

• Deliberate failure test: Broke app/handlers_test.go to verify PR gate blocks merge (red CI run: https://github.com/infernaltiger/DevOps-Intro/actions/runs/27550110703)
• Fix verification: Reverted the breakage, confirmed green CI and merge enabled (green run: https://github.com/infernaltiger/DevOps-Intro/actions/runs/27550434637)
• Branch protection: Configured on fork's main requiring vet, test, lint checks (screenshot in submissions/lab3.md)
• Performance measurement: Captured wall-clock times for baseline (63s), cached (46s), cached+matrix (50s), and bonus optimizations (49s)

Checklist

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