feat(idempotency): add idempotency support for multi-call detection

[ronaldtse]

Summary

• Add IdempotencyManager class to detect and skip redundant installations
• Add state file (.metanorma-setup-state.json) to track installation state
• Add checksum-based configuration change detection
• Add idempotent output to indicate skip vs reinstall
• Add integration test workflow for idempotency scenarios
• Add unit tests for IdempotencyManager

Details

When the action is called multiple times with the same configuration, it now detects the existing installation and skips the redundant setup. Configuration changes (version, method, platform) trigger reinstallation.

New Files

• src/idempotency/ - Idempotency module
  • index.ts - Public API exports
  • types.ts - Type definitions
  • idempotency-manager.ts - Main idempotency logic
• __tests__/idempotency.test.ts - Unit tests
• .github/workflows/idempotency-test.yml - Integration test workflow

Modified Files

• src/main.ts - Added idempotency check before installation
• action.yml - Added idempotent output

Test Plan

• Unit tests pass locally
• Integration test workflow passes on all platforms (Linux, macOS, Windows)
• Idempotency test verifies second call skips installation
• Config change test verifies reinstallation happens