Feature Request

Support broad file attachments in sessions_spawn (not only images), including the ability to pass archives/datasets for review workflows.

Why

Current discussions and implementations often focus on image transfer, but image is only a special case of a more general need: passing files between sessions/subagents.

For real workflows, agents need to receive:

• text/code files,
• documents,
• structured data,
• and sometimes compressed archives (.zip, .tar.gz) containing multiple files for review.

Use Cases

1. Code/Data review bundles
   • Main agent stages a review bundle and spawns a subagent/reviewer with attached archive.
2. Document analysis
   • Attach PDFs/CSVs/JSON for extraction and summarization.
3. Multi-file task delegation
   • Send a small project snapshot, config files, logs, or manifests.

Scope Proposal

Add/standardize a general attachments capability for sessions_spawn with:

• support for arbitrary file types (with explicit safety/size limits),
• multi-file payloads,
• optional archive payloads,
• clear materialization path + manifest in child workspace,
• robust cleanup policy (keep / delete),
• transcript redaction for sensitive inline content.

Relationship to Existing Image Request

Image support is an important subset of this broader capability.

Related issue (image-specific): #16344

This issue proposes treating image transfer as one concrete instance of a general file-attachment model.

Expected Outcome

A consistent, secure, and extensible attachment pipeline for sessions_spawn that enables image, document, and archive-based workflows without adding one-off APIs per media type.