Improve error messages in `forgekit new` when template is not found

[SubhanshuMG]

Summary

When a user runs forgekit new my-app --template unknown-template, the current error message is not helpful enough.

Expected behavior

Error: Template 'unknown-template' not found.

Available templates:
  web-app       Node.js + React + TypeScript
  api-service   Python + FastAPI + PostgreSQL
  ml-pipeline   Python + Jupyter + MLflow

Or load a custom template:
  forgekit new my-app --template github:owner/repo

Where to look

• packages/cli/src/commands/new.ts
• packages/cli/src/core/template-resolver.ts

Acceptance criteria

• Error message lists all available templates
• Error message hints at custom template loading
• Unit test added for the error case