ci: Add GitHub Actions CI workflow

[rianjs]

Summary

The repo has zero CI/CD — no GitHub Actions workflows. We need a basic CI workflow so tests run automatically on PRs and pushes to main.

Proposed Workflow

.github/workflows/ci.yml:

name: CI

on:
  push:
    branches: [main]
  pull_request:
    branches: [main]

jobs:
  test:
    runs-on: ubuntu-latest
    strategy:
      matrix:
        python-version: ["3.11", "3.12", "3.13"]
    steps:
      - uses: actions/checkout@v4
      - uses: actions/setup-python@v5
        with:
          python-version: ${{ matrix.python-version }}
      - run: pip install -r requirements.txt
      - run: pytest test_server.py -v

Design Decisions

• Python 3.11-3.13 matrix: 3.10 is EOL Oct 2026, not worth the CI minutes
• No linting for now: Keep it simple — can add ruff later
• No pip caching: Only 2 deps (mcp, pytest), installs fast enough
• ubuntu-latest only: This is a server, not a cross-platform binary

Context

PR #3 adds test_server.py with pytest tests. Once that merges, this workflow will ensure tests run on every future PR.