bug(discord): @mentioned bot ignored in another bot's thread — multi-bot same-channel

[thepagent]

Description

When two bots share a Discord channel and a user @mentions both in a single message, only the first bot responds. The second bot sees the message inside the first bot's thread and drops it because allow_user_messages = "involved" rejects messages in threads where the bot hasn't participated.

User: @BotA @BotB HIHI  (main channel)
  │
  ├─► BotA: sees main channel msg → creates thread → responds ✅
  │
  └─► BotB: sees msg inside BotA's thread → "not involved" → drops ❌

Discord converts the original message into a thread parent once BotA creates a thread. By the time BotB processes it, the message is no longer in the main channel — it's in BotA's thread.

Same issue when @mentioning BotB alone in BotA's thread — BotB ignores it because it hasn't participated, even though it's explicitly @mentioned.

Steps to Reproduce

1. Deploy two bots (e.g. AgentBroker + AgentDealer) in the same Discord channel
2. Set allow_user_messages = "involved" (default) on both
3. Send @AgentBroker @AgentDealer HIHI in the main channel
4. Observe: only AgentBroker responds, AgentDealer silently drops the message

Expected Behavior

If a bot is explicitly @mentioned (is_mentioned = true), it should always process the message regardless of thread ownership or participation status. The is_mentioned check should take priority over the Involved mode's thread participation check.

Suggested fix in discord.rs:

// Current (broken): is_mentioned is only checked at the top level
if !is_mentioned {
    match allow_user_messages {
        AllowUsers::Mentions => return,
        AllowUsers::Involved => {
            // rejects if bot not involved in thread
        }
    }
}

// Fix: always process if explicitly @mentioned
if is_mentioned {
    // proceed — explicit mention overrides thread participation check
} else {
    match allow_user_messages { ... }
}

Related:

• PR feat: per-thread serialization, conditional streaming, and multi-bot fixes #420 (multi-bot fixes, conditional streaming)
• Issue 🐍《華西街蛇女》— B2B AI 共創五言律詩接龍故事 #440 (multi-bot is_mentioned false positive — fixed)
• Issue bug(discord): rollout restart causes silent Gateway session conflict — bot stops receiving events #455 (Gateway session conflict on rollout restart)