feat(discord): include reply and forwarded message context in prompt

[BCDel89]

Summary

• Injects replied-to message content into the Claude prompt as [In reply to <user>]: <text>
• Injects forwarded message snapshot content as [Forwarded message from <user>]: <text>
• Attachments on referenced/forwarded messages are noted by filename but not downloaded
• Adds DiscordMessageSnapshot interface and extends DiscordMessage with message_reference and message_snapshots fields

Test plan

• Reply to a message in a Discord guild channel — verify the bot's response reflects the replied-to content
• Forward a message into a channel where the bot is listening — verify the forwarded content appears in the bot's context
• Send a plain message (no reply, no forward) — verify no regression
• Reply to a message that has attachments — verify attachment filenames are noted in the prompt

Closes BCD-89

[TerrysPOV]

Reviewed the current PR head against master. I did not find any blocking issues.

The reply and forwarded-message context is added only as prompt context, and referenced/forwarded attachments are noted by filename without being downloaded, which keeps the change appropriately scoped.

Local validation passed: git diff --check origin/master...HEAD, bun install, bun test, and bunx tsc --noEmit --pretty false.

Looks merge-ready from my side and will go in with the next cherry pick bulk merge.

[TerrysPOV]

sorry approved.