Change: Use model-driven tools for session attachment RAG

Why

Session attachment RAG currently pre-queries uploaded files before model generation and injects retrieved evidence into the latest user message. That limits the model's ability to rewrite queries, use multi-turn context, and decide whether retrieval is necessary.

What Changes

• Remove automatic pre-query evidence injection from the generation path.
• Expose session attachment RAG as a model toolset whenever large retrieval attachments are present and the model supports file tools.
• Add lightweight attachment tags and reminders for retrieval-mode files so the model notices uploaded files without receiving full content.
• Add a PoC model behavior evaluation harness with synthetic document fixtures in a separate local repository.

Impact

• Affected specs: session-attachment-rag
• Affected code:
  • src/shared/context/builder.ts
  • src/renderer/stores/session/tools-builder.ts
  • src/renderer/stores/session/orchestration.ts
  • src/renderer/packages/model-calls/toolsets/session-attachment-rag.ts
  • scripts/session-rag-eval/