Route project media generation through Summer Studio

[TimofeySukh]

Summary

• add a Studio-only media generation boundary to the core Summer workflow and asset router
• prohibit Python, GDScript, shell, native chat, and external-generator fallbacks for authored project media
• require durable Summer asset imports and stop with a Studio dashboard handoff when generation tools are unavailable
• add the same invariant to the MCP agent playbook and cover it with a regression test

Verification

• npm test — 47 test files, 433 tests passed
• npm run build
• git diff --check origin/main...HEAD

Scope

Prompt-only behavior change. No generator implementation, tool permissions, or runtime enforcement changed.