feat(create-gen-app): replace .questions.json with .boilerplate.json format

[pyramation]

Summary

This PR replaces the legacy .questions.json/.js configuration format with the new .boilerplate.json/.js format for template scaffolding. It also switches tests to use a dedicated testing boilerplate repository for isolation.

Key changes:

• Breaking: Removed support for .questions.json and .questions.js files
• Added support for .boilerplate.json and .boilerplate.js as the new configuration format
• Skip .boilerplate.json, .boilerplate.js, and .boilerplates.json from being copied to generated output
• Changed test default repo from pgpm-boilerplates (production) to pgpm-boilerplates-testing
• Updated tests and README documentation

Updates since last revision

• Fixed failing CI by updating create-gen-app-test snapshots to reflect that .boilerplate.json is now correctly excluded from generated output
• Snapshots also updated to reflect test:watch script change in the testing boilerplate repo (jest --watch → jest --watchAll)

Review & Testing Checklist for Human

• Verify the testing repo exists: Confirm https://github.com/constructive-io/pgpm-boilerplates-testing.git is set up and has the expected template structure
• Check constructive repo compatibility: The template-scaffold.ts in constructive reads .boilerplate.json directly - verify it still works with these changes
• Test with a real template: Run pgpm init with a template that uses .boilerplate.json to verify end-to-end functionality
• Verify no production templates still use .questions.json: Since this is a breaking change, ensure all boilerplate repos have been migrated

Recommended test plan:

1. Clone a workspace using pgpm init workspace
2. Create a module with pgpm init using a template from the boilerplates repo
3. Verify the generated output does NOT contain .boilerplate.json or .boilerplates.json files

Notes

• The user explicitly requested no backward compatibility for .questions.json
• The test repo change allows intentional test breakage without affecting production templates

Link to Devin run: https://app.devin.ai/sessions/718276b3817a4c70831019b07be99855
Requested by: Dan Lynch (@pyramation)

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