feat(discord): preserve user mention UIDs for LLM mention-back

[chaodu-agent]

Summary

Closes #460

Simplify resolve_mentions() to keep other-user <@UID> mentions as-is instead of resolving them to @DisplayName (which discards the UID). This lets the LLM copy <@UID> into its reply for correct Discord mention rendering — no outbound transformation needed.

Changes (src/discord.rs, +6/-24)

• Remove step 2 — no longer resolves <@UID> → @DisplayName. User mentions pass through as raw <@UID>.
• Remove USER_MENTION_RE — no longer needed since user mentions are preserved.
• Remove mentions parameter — resolve_mentions no longer needs the serenity User list.
• Call unconditionally — not just when is_mentioned, so thread follow-ups also get bot-mention stripping.

What stays the same

• Bot own mention: stripped
• Role mentions: @(role) fallback
• send_message / outbound: untouched

Why preserve raw <@UID> instead of <@UID|DisplayName>

We explored <@UID|DisplayName> (Slack-style) but LLMs don't reliably preserve token formats — the model might respond with @Name, <@Name>, or <@UID|Name>. Only raw <@UID> is safe:

• LLM copies <@UID> → Discord renders clickable mention ✅
• LLM writes @Name → plain text, no worse than before ⚠️
• No outbound regex needed — zero risk of mangling

Recommended usage: ice-breaking in multi-bot chatrooms

Since <@UID> is now preserved, bots need a UID→name mapping to know who is who. The recommended approach is to send an ice-breaking greeting at the start of a multi-bot chatroom (via system prompt or context entry):

We have 3 people in this room:

ME @MY_NAME
BOT_A_NAME @BOT_A
BOT_B_NAME @BOT_B

Make sure to use <@UID> to mention someone in the messages.

This lets each bot build the UID→name mapping in its own context from the very beginning and learn how to mention others correctly.

Design notes

Slack isolation: Slack has its own independent mention handling in src/slack.rs. Not touched.

Thread title: shorten_thread_name() will show raw <@UID> in thread titles. Pre-existing gap (old code stripped mentions entirely), can be addressed separately.

DC: https://discord.com/channels/1491295327620169908/1494739741600387204/1495368292146217071

[thepagent]

LGTM — clean implementation.

• Inbound resolve_mentions() now runs unconditionally (good: thread follow-ups also get enrichment)
• Outbound strip_mention_names() correctly applied in both streaming and final send paths
• Regex safety verified: USER_MENTION_RE won't clobber resolved <@UID|Name> mentions
• No Slack impact (separate code paths)
• Empirically confirmed Discord does not render <@UID|Name> (see #460 comment)

Minor note: for thread follow-ups without explicit @mention, msg.mentions may be sparse — unresolved UIDs fall through to @(user) which is acceptable.