feat: outbound image/file attachments from agent → Discord

[DrVictorChen]

Description

Support sending images and files from the agent back to Discord as native attachments. Currently OpenAB only supports inbound attachments (Discord → agent via base64 ImageContent blocks, PR #158), but the outbound path (edit_message) only writes .content(text) with no CreateAttachment.

Use Case

• Agent captures a screenshot (e.g. via peekaboo on macOS) and posts it back to the thread
• Agent generates a chart, diagram, or report and shares it visually
• Agent produces a file (CSV, PDF) and attaches it to the reply

Current workaround: The agent calls the Discord REST API directly via curl with the bot token to POST multipart form data. This works but is hacky — agent needs the bot token and channel ID, messages land outside OpenAB's thread context, and it's not portable across agents.

Proposed Solution

Detection: Agent response contains ![alt](/path/to/file) markers (markdown image syntax, natural for LLM agents).

Handling in discord.rs:

1. Parse agent response for file markers via regex
2. Validate: file exists, size ≤ 25 MB (Discord limit), path in allowlisted directories only (security)
3. Upload via serenity CreateAttachment::path()
4. Strip markers from text content
5. Send attachments as follow-up messages

Security:

• Path allowlist (e.g. /tmp/, agent working dir) to prevent exfiltration
• Size cap at Discord's 25 MB limit
• Rate limit on attachments per message

Environment: OpenAB v0.7.2, macOS + launchd multi-agent (Claude/Cursor/Codex/Copilot). Confirmed with maintainer on Discord that PRs are welcome.

Happy to submit a PR after feedback on detection format and security model.

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