feat: include Discord reply/quote context in agent prompt

[Agent-Mouses]

Description

When a user replies to (quotes) a message inside a bot thread using Discord's reply function, the bot only receives the new message text. The quoted/referenced message content is lost — the agent has no idea what "this" refers to.

Current Behavior

• User quotes an earlier message and types "summarize this"
• OpenAB sends only "summarize this" to the ACP agent
• Agent has no context about what was quoted

Expected Behavior

• When msg.referenced_message is present, OpenAB should fetch the referenced message content and prepend it to the prompt (or include it as a separate context block)
• Example prompt sent to agent:

[Quoted message from @user]:
<content of the quoted message>

summarize this

Implementation Notes

• Discord's Message struct has message_reference and referenced_message fields
• referenced_message is often populated by the gateway, but may need a fallback API call via channel_id.message(http, message_id)
• This is especially useful for multi-agent collaboration where a user quotes a bot's response to ask another bot about it
• Should work for both in-thread replies and channel-level replies that trigger thread creation

[github-actions]

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