feat: add controlled MVP template registry

[dDevAhmed]

Summary

Implements CLI-025 by introducing a closed, typed registry for the four supported StellarForge MVP templates and wiring template selection into stellarforge new.

Included

• controlled template IDs: basic-app, full-stack, smart-contract, api-service
• typed template names/descriptions
• registry listing and lookup helpers
• basic-app as the default selection
• new --template <id> forwarding through validated generator input
• safe rejection of unknown IDs, paths, and URLs
• no source path or remote URL fields in registry metadata
• unit coverage for registry contents, default/explicit selection, unknown values, and CLI option forwarding

Security

• no arbitrary filesystem template path loading
• no remote URL loading
• no plugin resolution
• unknown identifiers fail with a validation error before generation

Actual bundled template file copying remains CLI-026 and individual template content remains CLI-027 through CLI-030.

Closes #28