feat(create-gen-app): add TemplateScaffolder orchestrator and prompter pass-through

[pyramation]

Summary

This PR adds two major features to create-gen-app:

1. Optional prompter parameter - Allows callers to pass an existing Inquirerer instance to Templatizer.process() instead of creating a new one internally. This addresses the "two instances on stdin" problem that causes double-echoed keystrokes.

2. TemplateScaffolder orchestrator class - A high-level API that combines CacheManager, GitCloner, and Templatizer into a single, easy-to-use class. This was the "missing top-level API" that downstream tools like pgpm were reimplementing.

Ownership semantics for prompter:

• If prompter is provided: caller retains ownership and must close it
• If prompter is not provided: a new instance is created and closed automatically (existing behavior)

New promptUser signature:

promptUser(extractedVariables, argv?, existingPrompter?, noTty?)

Updates since last revision

• Added inspect() method to TemplateScaffolder that clones/caches a template and reads its .boilerplate.json configuration without scaffolding
• New InspectOptions and InspectResult types exported from scaffolder module
• This enables metadata-driven workflows where callers can read the template's type field before deciding how to handle scaffolding (e.g., pgpm can now use pgpm init <anything> and branch based on template type)
• Added inspectLocal() and inspectRemote() private methods that mirror the scaffold methods but skip the templatizer step

Review & Testing Checklist for Human

• inspect() method correctness: The inspectLocal and inspectRemote methods duplicate cache/clone logic from scaffoldFromLocal/scaffoldFromRemote - verify they stay in sync and both correctly resolve paths via .boilerplates.json
• No tests for inspect(): The new method was added without dedicated tests - consider if this is acceptable or if tests should be added before merge
• TemplateScaffolder path resolution: Verify the resolveFromPath logic correctly handles: (1) direct paths, (2) .boilerplates.json dir-based resolution, (3) fallback behavior
• No pgpm-specific code: Ensure no pgpm-specific strings/constants leaked into create-gen-app (e.g., "workspace", "module", "PGPM_*")
• Prompter threading: Verify TemplateScaffolder.scaffold() correctly passes the prompter through to Templatizer.process()

Test plan:

1. After publishing a new version, test with the companion PR in constructive repo that updates pgpm/cli to use inspectTemplate() for metadata-driven init
2. Run pgpm init workspace and pgpm init module end-to-end - verify the template's type field is correctly read and the appropriate flow is triggered
3. Test both local template paths and remote git URLs with caching

Notes

This PR is part of a two-PR solution:

• PR 1 (this): Add TemplateScaffolder, inspect() method, and prompter pass-through to create-gen-app
• PR 2 (constructive): Refactor pgpm init to single entry point using metadata-driven flow

Important: The constructive PR depends on the inspect() method added here. A new version of create-gen-app must be published before the constructive PR can build.

Link to Devin run: https://app.devin.ai/sessions/48555739db3d4f1b9e9edfa9ea1e559a
Requested by: Dan Lynch (@pyramation)

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