perf(discord): eager multibot detection from msg.author

[chaodu-agent]

Summary

Builds on #464. Eliminates unnecessary API fetches in multibot-mentions mode.

Problem

In #464, bot_participated_in_thread() fetches up to 200 messages on every unmentioned message in MultibotMentions mode — even in single-bot threads where no other bot will ever be found. This is because cached_involved = true could not short-circuit without knowing other_bot_present.

Solution

Detect multi-bot threads eagerly from msg.author instead of fetching history:

// Before gating logic — zero API call
if in_thread && msg.author.bot && msg.author.id != bot_id {
    cache multibot_threads[thread_id] = now
}

When a bot message arrives in a thread, we already know it is multi-bot. No fetch needed.

This allows bot_participated_in_thread() to short-circuit on cached_involved alone:

// Before (fetched every time for MultibotMentions):
if cached_involved && cached_multibot { return (true, true); }
if cached_involved && mode != MultibotMentions { return (true, false); }
// ↑ MultibotMentions with cached_involved but !cached_multibot → fetch

// After (always short-circuits):
if cached_involved { return (true, cached_multibot); }
// ↑ cached_multibot is populated eagerly from msg.author

Impact

Scenario	Before	After
Single-bot thread, unmentioned msg	Fetch 200 msgs every time	Zero fetch (cache hit)
Multi-bot thread, after detection	Zero fetch (cache hit)	Zero fetch (cache hit)
First bot message in thread	Fetch (to check involvement)	Fetch (same) + cache multibot

Cold-start note

After bot restart, both caches are empty. If a thread is already multi-bot but the next message is from a user (not a bot), early detection won't trigger and cached_multibot = false. However, cached_involved is also false (cleared on restart), so bot_participated_in_thread will fetch — and the fetch includes other_bot_present detection, which repopulates the cache. This is safe and consistent with #464 behavior — just one fallback fetch on the first message after restart.

Changes (src/discord.rs, +12/-6)

• Add early multibot detection from msg.author before gating logic
• Simplify bot_participated_in_thread: cached_involved → return (true, cached_multibot) immediately, no mode-specific branching

[github-actions]

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