Issue: Consolidate Site Loading Logic into API

Summary

Both the CLI and TUI implement their own site loading mechanisms. This logic should be consolidated into a shared API module to ensure consistency and reduce duplication.

Current State

CLI Implementation (render_engine_cli.utils)

• get_site(import_path: str, site: str, reload: bool) - imports module and returns Site
• split_module_site(module_site: str) - parses "module:site" format
• Loads from dynamically imported Python modules

TUI Implementation (render_engine_tui.site_loader)

• SiteLoader class with caching
• Reads from pyproject.toml [tool.render-engine.cli] section
• Provides load_site(), get_collections(), get_collection(), reload_site()
• More robust error handling with detailed exceptions

Proposed API

Create render_engine_api.site_loader with:

class SiteLoader:
    """Unified site loading for CLI, TUI, and other tools."""

    def __init__(self, project_root: Optional[Path] = None):
        """Initialize from project root or cwd."""

    def load_site(self) -> Site:
        """Load Site from pyproject.toml configuration."""

    def get_collections(self) -> Dict[str, Collection]:
        """Get all Collections from the Site."""

    def get_collection(self, slug: str) -> Optional[Collection]:
        """Get a specific Collection by slug."""

    def reload_site(self) -> None:
        """Force reload Site from disk."""

Benefits

1. Single Source of Truth: One implementation for all tools
2. Better Error Handling: TUI's implementation has comprehensive error messages
3. Caching: Built-in caching reduces redundant imports
4. Testing: Easier to test one implementation
5. Future Tools: New tools can use the same loader

Migration Path

1. Move SiteLoader from TUI to API package
2. Add split_module_site() helper from CLI
3. Update CLI to use render_engine_api.site_loader.SiteLoader
4. Update TUI to import from API instead of local implementation
5. Add tests in API package

Dependencies

• Must support both "module:site" string format (CLI) and pyproject.toml reading (TUI)
• Should handle both absolute and relative imports
• Need proper sys.path manipulation for project imports

Related Issues

• configuration management via toml #2: Configuration Management API
• CREATE: Collection Adapter #3: Collection Operations API