feat(discord): per-thread streaming decision based on multibot detection

[chaodu-agent]

Description

Streaming (edit-based live updates) should be decided per-thread based on multibot detection, not globally based on allow_bot_messages config.

Current Behavior

allow_bot_messages = "off"      -> streaming ON (all threads)
allow_bot_messages = "mentions"  -> streaming OFF (all threads)

A human chatting 1-on-1 with a bot gets no streaming just because the deployment allows bot-to-bot interaction.

Expected Behavior

Default streaming ON. Only disable when 2+ bots detected in the thread.

fn should_stream(other_bot_present: bool) -> bool {
    !other_bot_present
}

Scenarios

Scenario 1: Single bot, human conversation

1. Human sends @bot-A hello in channel
2. openab creates new thread
3. other_bot_present = false (new thread, no other bots)
4. streaming ON - bot-A responds with live edit updates
5. Human sends follow-up (no mention) - bot-A still alone - streaming ON

Scenario 2: Second bot joins mid-conversation

1. Thread has bot-A + human (streaming ON)
2. Human sends @bot-B can you help too?
3. bot-B checks thread, sees bot-A -> other_bot_present = true
4. bot-B responds with send-once (no streaming)
5. multibot_threads cache updated for this thread
6. Human sends @bot-A what do you think?
7. bot-A checks thread, sees bot-B -> other_bot_present = true
8. bot-A switches to send-once (was streaming before, now off)

Scenario 3: Channel message without mention

1. Human sends hello everyone in channel (no @mention)
2. Bot ignores - no streaming decision needed

Scenario 4: Bot-to-bot conversation

1. bot-B sends @bot-A please review this in thread
2. bot-A checks thread -> other_bot_present = true
3. send-once (bot-to-bot always send-once)

Use Case

Users deploying openab with allow_bot_messages=mentions for multi-agent collaboration lose streaming in ALL threads, including 1-on-1 human conversations. This degrades the human UX unnecessarily. Per-thread streaming lets the same deployment provide live edit updates for single-bot threads while safely using send-once in multi-bot threads.

Implementation Notes

The data is already there:

• multibot_threads cache tracks which threads have multiple bots
• other_bot_present is computed per-thread in bot_participated_in_thread()
• use_streaming() needs to change from per-adapter global to per-thread

The allow_bot_messages config should have no effect on streaming decisions.

Background

• 0.7.3: streaming always on (hardcoded in discord.rs)
• 0.8.0: streaming accidentally removed during Slack adapter refactor
• fix(discord): restore edit_message and use_streaming in ChatAdapter #503: restored use_streaming()=true unconditionally
• fix(discord): conditional streaming parity with Slack #533: changed to conditional based on allow_bot_messages config (global, not per-thread)

[chaodu-agent]

Simplified Decision

Default streaming ON. Only disable when 2+ bots detected in the thread:

fn should_stream(other_bot_present: bool) -> bool {
    !other_bot_present
}

No dependency on allow_bot_messages config. The multibot_threads cache already tracks this per-thread.