Testing: CI workflow for automated test runs

[fegge]

Parent issue: #90

Overview

Set up GitHub Actions to run tests on PRs and pushes.

Tasks

• Create .github/workflows/test.yml
• Configure Xvfb for extension-host tests on Linux
• Set up VS Code version matrix (stable, insiders)
• Upload coverage reports to Codecov
• Add status badge to README

Workflow Template

name: Tests
on: [push, pull_request]

jobs:
  test:
    runs-on: ubuntu-latest
    strategy:
      matrix:
        vscode-version: [stable, insiders]
    steps:
      - uses: actions/checkout@v4
      - uses: actions/setup-node@v4
        with:
          node-version: 20
      - run: npm ci
      - run: xvfb-run -a npm test
        env:
          DISPLAY: ":99"
      - run: npm run coverage
      - uses: codecov/codecov-action@v3

Acceptance Criteria

• Tests run on every PR
• Coverage reports uploaded
• Badge visible in README
• Failed tests block merge