docs: add sendimages.md and sendfiles.md

[chaodu-agent]

Summary

Add two best-practice guides for AI agents to send images and files back to Discord (and other platforms).

Architecture

Direct upload (images & small files)

┌──────────┐  text only   ┌──────────┐  ACP stdio   ┌──────────────┐
│  Discord  │◄────────────│  OpenAB   │◄────────────│  Agent (CLI)  │
│  Thread   │             └──────────┘              └──────┬───────┘
│           │                                              │
│           │         Discord REST API                     │
│           │◄─────────────────────────────────────────────┘
│           │  POST /channels/{thread_id}/messages
│           │  + multipart file attachment
└──────────┘

Enterprise / large files (presigned URL)

┌──────────┐  text only   ┌──────────┐  ACP stdio   ┌──────────────┐
│  Discord  │◄────────────│  OpenAB   │◄────────────│  Agent (CLI)  │
│  Thread   │             └──────────┘              └──────┬───────┘
│           │                                              │
│           │  send presigned URL as message                │  upload file
│           │◄─────────────────────────────────────────────┤─────────────►┌─────┐
│           │  POST /channels/{thread_id}/messages         │              │ S3  │
└─────┬────┘                                               │              │ R2  │
      │                                                    │              │ GCS │
      │  user clicks link                                  │              └──┬──┘
      └────────────────────────────────────────────────────────────────────►│
                              presigned GET                                 │
                              ◄─────────────────────────────────────────────┘

New files

• docs/sendimages.md — How to send images back to Discord

  • sender_context → thread_id flow
  • curl / Python / Node.js examples
  • Sidecar file-watcher pattern (as demonstrated by community)
  • Security considerations & bot permission checklist

• docs/sendfiles.md — How to send files back to Discord

  • Single & multi-file upload (up to 10 attachments)
  • File size limits by server boost level
  • Enterprise best practice: upload to S3/R2/Google Drive → presigned URL → send link (no size limit, data stays off Discord, you control TTL)
  • Common file types table
  • Security considerations & bot permission checklist

Context

From community discussion: OAB itself streams text only via ACP JSON-RPC. These docs enable agents to learn how to send images/files out-of-band by reading the documentation — no OAB code changes needed.

What was tested

• Docs reviewed for accuracy against Discord API v10 and OAB SenderContext struct
• All code examples verified against Discord API docs

[github-actions]

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Discord Discussion URL: https://discord.com/channels/...

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