Extend CI caching with .NET tools, Playwright browsers, and Docker layer cache

[Copilot]

Issue #12 implemented NuGet package caching in PR #33. This PR completes the optimization with additional caching layers mentioned in the original issue.

Changes

Caching additions:

• .NET tools (~/.dotnet/tools) - global tools like Playwright CLI
• Playwright browsers (~/.cache/ms-playwright) - 200-500MB browser binaries
• Docker build cache - GitHub Actions cache backend with max mode for layer reuse

Workflow updates:

• build-and-test.yml - added tools and Playwright browser caches
• dotnet.yml - added tools cache to security-scan and code-quality jobs
• deploy.yml - added Buildx setup with GHA cache backend

Documentation:

• docs/ci-cd.md - pipeline overview, caching strategy, performance metrics, debugging guide

Impact

Expected CI time reduction: ~5-10 minutes per run

• Tool installation: ~90% faster (cached vs download/install)
• Playwright browsers: ~80-90% faster (cached vs download 500MB)
• Docker builds: ~50-70% faster (layer reuse vs full rebuild)

# Example cache configuration added
- name: Cache .NET tools
  uses: actions/cache@v4
  with:
    path: ~/.dotnet/tools
    key: ${{ runner.os }}-dotnet-tools-${{ hashFiles('**/*.csproj', '**/*.sln') }}
    restore-keys: |
      ${{ runner.os }}-dotnet-tools-

Original prompt

can you continue the work I started earlier on pr 12?

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