Export approved Studio revisions as reviewable eval candidates

[anandpant]

Goal

Turn an explicitly approved Studio revision and its source artifact into a portable, reviewable eval candidate without automatically promoting user content into maintained evals.

Current baseline

• Maintained scenarios are checked into packages/diagram/scenarios and run through apps/eval-harness.
• Studio has no approved-revision export contract today.
• Code Mode artifacts retain a source-artifact relationship, but that alone does not define consent, review, or promotion into the maintained scenario suite.

Scope

• Define an eval-candidate format that retains the before/after artifacts, source relationship, diagram type, and available generation metadata.
• Add an explicit export path for a Studio revision only after both revision approval and a separate recorded consent decision; approval alone does not authorize export or reuse.
• Define the human review/import boundary between candidates and maintained scenarios.
• Document consent, access, retention, and deletion expectations before approved user content can be reused.

Non-goals

• Automatically learning from every edit.
• Automatically changing prompts, renderers, or models.
• Treating approval as consent for public or indefinite reuse.

Acceptance criteria

• An approved revision can be exported with a verifiable source/edited pair and provenance.
• The export is portable and reviewable without depending on live Studio records.
• Import into maintained evals is explicit and does not happen automatically.
• Privacy, retention, access, and deletion behavior is documented and tested where enforceable.
• At least one approved candidate can be reviewed and exercised through the current scenario/eval-harness workflow.

Current touchpoints

• packages/studio/projects/
• packages/diagram/agent/src/lib/code-mode-artifacts.ts
• packages/diagram/scenarios/src/lib/scenarios.ts
• apps/eval-harness/

This replaces the eval-loop portion of #63 and depends on durable Studio revision/approval in #282 plus the consent policy and record needed for candidate export.