fix(discord): multibot-mentions fails to detect other bots due to gating order

[thepagent]

Description

multibot-mentions mode fails to detect multi-bot threads when allow_bot_messages = "mentions". The bot responds without @mention even after another bot has posted in the thread.

Root cause: Eager multibot detection (line 335) runs after bot message gating (line 248). When another bot posts without @mentioning this bot, the message is discarded at the gating step and never reaches the detection code.

Bot gating (line 248)              ← other bot msg filtered here
  → return if bot msg
...
Eager multibot detection (line 335) ← never reached
  → multibot_threads.insert(key)

https://github.com/openabdev/openab/blob/9b7680d/src/discord.rs#L248

https://github.com/openabdev/openab/blob/9b7680d/src/discord.rs#L335

Steps to Reproduce

1. Set allow_bot_messages = "mentions" and allow_user_messages = "multibot-mentions"
2. Human: @BotA HIHI → BotA creates thread, responds
3. Human: @BotB 你來 → BotB responds in the thread
4. Human: nice (no @mention) → BotA responds ← should not respond

Expected Behavior

After step 3, BotA should detect the thread is multi-bot and require @mention for step 4.

Fix

Move eager multibot detection before bot message gating:

// BEFORE bot gating — detect other bots from every incoming message
if in_thread && msg.author.bot && msg.author.id != bot_id {
    let key = msg.channel_id.to_string();
    let mut cache = self.multibot_threads.lock().await;
    cache.entry(key).or_insert_with(tokio::time::Instant::now);
}

// Bot message gating (unchanged)
if msg.author.bot { ... }

Zero API calls, zero fetch — just check msg.author.bot on every incoming message before any gating.