Match the submit-a-plugin entry example to the marketplace schema

[ylcn91]

Human comments

What was wrong

The submit-a-plugin skill's worked marketplace entry still carried an
engines object. The published entry contract sets
additionalProperties: false and never defined engines: a listing
declares no compatibility, and bb reads engines.bb and
engines.bbPluginSdk from the plugin's own package.json instead, as
docs/plugin-marketplace-plan.md describes. Anyone who copied the
example produced an entry that npm run build and npm run check
reject in get-bb/marketplace. Nothing guarded the example against the
schema, so the field survived the schema change.

Issue: #2508. Investigation report: https://get-bb.github.io/reports/issues/2508.html

What changed

• references/marketplace-entry.md: drops engines from the worked
  entry, and replaces the "copy honest engine ranges" prose with the
  actual rule, that compatibility lives in the plugin manifest.
• references/pull-request.md: drops the "Entry engine ranges do not
  exceed manifest ranges" review item, which checked a field an entry
  cannot have.
• test/skills/submit-a-plugin.test.ts: validates the worked entry
  against apps/web/public/schemas/marketplace-v2.schema.json, the
  document get-bb/marketplace compiles each entry file against.

The remaining example fields (id, displayName, description,
icon, tags, author, source) already match the schema.

How you verified

• pnpm exec turbo run test --filter=@bb/server -- test/skills/submit-a-plugin.test.ts
  passes. Re-adding engines to the example fails the new test with
  must NOT have additional properties.
• pnpm exec turbo run typecheck --filter=@bb/server passes.
• Validated the corrected example against schema/marketplace-v2.schema.json
  fetched from get-bb/marketplace@main; that file is byte-identical to the
  copy this repo publishes.

Fixes #2508

AGENT GENERATED