Add comprehensive CI workflow for MistDemo example

[leogdion]

Description

MistDemo is currently the only example in the Examples/ directory without its own comprehensive CI workflow. BushelCloud and CelestraCloud both have workflows in their .github/workflows/ directories with:

• Multi-OS testing (Ubuntu, macOS, Windows)
• Multi-Swift version matrices (6.2 stable + 6.3 nightly)
• Code coverage reporting via Codecov
• Platform-specific testing

Current State

MistDemo is tested via the main repo's .github/workflows/examples.yml, which provides only basic smoke testing:

• Ubuntu-latest only
• Swift 6.2 only
• No coverage reporting
• No macOS-specific validation

Proposed Workflow

Add Examples/MistDemo/.github/workflows/MistDemo.yml with:

Matrix Strategy:

matrix:
  include:
    # Ubuntu builds
    - os: ubuntu-latest
      swift: 6.2
      container: swift:6.2-noble
    - os: ubuntu-latest
      swift: 6.3
      container: swiftlang/swift:nightly-6.3-noble
      nightly: true

    # macOS build
    - os: macos-latest
      swift: 6.2

Features:

• Build and test on Ubuntu (noble container) and macOS
• Test with Swift 6.2 stable and 6.3 nightly
• Code coverage reporting to Codecov with flag mistdemo
• Test ConfigKeyKit library separately
• Validate CLI executable builds

Reference Implementations:

• Examples/BushelCloud/.github/workflows/BushelCloud.yml (similar structure)
• Examples/CelestraCloud/.github/workflows/CelestraCloud.yml (more comprehensive)
• .github/workflows/MistKit.yml (main library workflow patterns)

Testing Configuration

Since MistDemo uses ConfigKeyKit and depends on:

• swift-configuration
• Hummingbird
• swift-service-lifecycle

The workflow should validate:

1. ConfigKeyKit library builds
2. MistDemoTests pass
3. mistdemo executable builds successfully
4. macOS-specific integration (since config files may behave differently on macOS)

Labels

Added: enhancement, testing