[WIP] Add prose to the proposal for moving blueprints off the release train

[Copilot]

Stacked on #1160. Adds prose requested in this review comment about allowing users to select any specific blueprint version at generation time—not just "bundled" or "latest"—via both an interactive version selection UI and the existing --blueprint @blueprint/name@version mechanism.

Propose No Release Train for Blueprints

Rendered

Summary

This pull request is proposing a new RFC.

To succeed, it will need to pass into the Exploring Stage, followed by the Accepted Stage.

A Proposed or Exploring RFC may also move to the Closed Stage if it is withdrawn by the author or if it is rejected by the Ember team. This requires an "FCP to Close" period.

An FCP is required before merging this PR to advance to Accepted.

Upon merging this PR, automation will open a draft PR for this RFC to move to the Ready for Released Stage.

Changes in this PR

• Goals — added bullet: users may specify any specific blueprint version at generation time—interactively via a version selection UI, or non-interactively via --blueprint @blueprint/name@version (e.g. for testing)
• Generation-time prompt — added a third interactive option "Choose a specific version" which presents a version selection UI (a list of recent published versions from the registry) so the user can pick an exact version
• Prompt text and behavior — added description of the "Specific version" option to the tradeoff list
• New subsection "Specifying a specific blueprint version" — documents both interactive (version selection list) and non-interactive (--blueprint @blueprint/name@version) approaches, covering use cases: pre-bundled testing, debugging/reproducing output, downgrading for comparison; notes that the interactive version selection UI is contingent on out-of-scope work to make blueprint generation generally interactive
• Examples — added examples for the interactive "Choose a specific version" flow and for ember new my-app --blueprint @ember/app-blueprint@1.2.3 (non-interactive, skips prompt)
• Non-interactive environments — notes that --blueprint @blueprint/name@version (the existing mechanism) can be used in automation to pin a specific version

Checklist to move to Exploring

• The team believes the concepts described in the RFC should be pursued.
• The label S-Proposed is removed from the PR and the label S-Exploring is added.
• The Ember team is willing to work on the proposal to get it to Accepted

Checklist to move to Accepted

• This PR has had the Final Comment Period label has been added to start the FCP
• The RFC is announced in #news-and-announcements in the Ember Discord.
• The RFC has complete prose, is well-specified and ready for implementation.
  • All sections of the RFC are filled out.
  • Any unanswered questions are outlined and expected to be answered before Ready for Release.
  • "How we teach this?" is sufficiently filled out.
• The RFC has a champion within one of the relevant teams.
• The RFC has consensus after the FCP period.

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[NullVoxPopuli]

@copilot we don't want this extra flag, I think we can already do --blueprint @blueprint/name@version

[Copilot]

Updated in 1371c47 — removed the custom --blueprints-version flag and the prose now references the existing --blueprint @blueprint/name@version mechanism instead.

[NullVoxPopuli]

@copilot we also want the interactive side of this to work as well. while --blueprint is how we want users to generate specific things in a non-interactive way, the interactive mode should provide a version selection UI for the selected blueprint

[Copilot]

Done in f76a2ae. The interactive prompt now includes a third option — "Choose a specific version" — which triggers a secondary version selection UI (a list of recent published versions fetched from the registry). The non-interactive path continues to use --blueprint @blueprint/name@version. Both are documented in the updated "Specifying a specific blueprint version" subsection.

[NullVoxPopuli]

@copilot add that this is contingent on out-of-scope work for making blueprint generation in general interactive

[Copilot]

Done in ef10415. Added a blockquote note directly after the interactive mode description clarifying that the version selection UI is contingent on out-of-scope work to make blueprint generation generally interactive, and that this prerequisite work is not defined by this RFC.