Add GitHub Actions CI workflow

[manavgup]

Context

Borrowed from IBM/mcp-context-forge which has modular workflows with concurrency control and path-based filtering.

FastStack has no CI — make check only runs locally.

What to do

• Add .github/workflows/ci.yml:
  • Trigger: push to main, PRs to main
  • Concurrency: cancel stale runs on same branch
  • Steps: make install → make check (lint + typecheck + test with coverage)
  • Python 3.12 matrix (expand to 3.13 later)
• Add branch protection rules requiring CI to pass before merge

Acceptance criteria

• PRs show green/red CI status
• make check runs on every push and PR