[Standalone/CLI] Promote standalone product generator to stable devkit command

[ogyrec-o]

Summary

Promote the standalone product generator from script/helper form into a stable devkit command.

#68 intentionally added a conservative script first, without freezing the final CLI surface too early. Once the product manifest, buildable generated repo, and product-boundary packaging contracts are clearer, the generator should become a stable command.

Motivation

The script is good for Stage 2a/2b, but the long-term author experience should be a supported devkit command with stable help text, validation, and docs.

The command should feel like a first-class product creation workflow, not an internal packaging script.

Possible command shape

Exact binary and subcommand are TBD.

Possible shapes:

• freven-devkit new-standalone-product
• freven_boot new-standalone-product
• freven_boot product new
• dedicated freven_product or freven_devkit helper crate

The issue should decide the correct public surface before freezing it.

Requirements

• Reuse or port current script validation behavior.
• Preserve dry-run / plan mode.
• Preserve deterministic output.
• Preserve zero-Vanilla default.
• Provide clear --help.
• Provide clear error messages for invalid identity.
• Support generated product.toml.
• Document command examples.
• Decide where the command lives:
  • freven_boot
  • a dedicated devkit tool crate
  • an external packaged helper
• Preserve packaged authoring profile availability.

Acceptance criteria

This is done when:

• the stable command exists
• docs show the supported command instead of only the script
• the script is either retained as compatibility wrapper or intentionally removed
• existing generator checks are ported or replaced
• packaged authoring profiles expose the stable command
• shipped standalone product profile does not expose authoring-only generator tools
• migration notes explain the relationship to scripts/new_standalone_product.sh

Non-goals

• Do not design store metadata here.
• Do not build editor/catalog/publishing backend here.
• Do not force product-owned packaging to be completed in this issue unless it is already ready.

Related context

• [Standalone/Templates] Implement standalone product repo generator scaffold #68