feat(discord): add multibot-mentions mode for allow_user_messages

[chaodu-agent]

Summary

Closes #463

Add a third mode for allow_user_messages that combines the best of involved and mentions:

• Single-bot threads: natural conversation, no @mention needed (same as involved)
• Multi-bot threads: requires @mention so only the addressed bot responds (same as mentions)

[discord]
allow_user_messages = "multibot-mentions"

Changes

src/config.rs

• Add MultibotMentions variant to AllowUsers enum
• Accept "multibot-mentions" or "multibot_mentions" in config deserialization

src/discord.rs

• Refactor bot_participated_in_thread() to return (involved, other_bot_present) tuple
  • Reuses the existing message fetch (up to 200 messages) — zero additional API calls
  • Checks m.author.bot && m.author.id != bot_id to detect other bots
  • For Involved mode with a cache hit, skips the fetch entirely (no regression)
• Add multibot_threads positive-only cache (same pattern as participated_threads)
  • Once a thread is detected as multi-bot, the result is cached — subsequent messages skip the fetch
  • Both caches hit → return (true, true), zero API call
  • Same eviction strategy (session TTL + capacity cap)
• Add MultibotMentions branch in the gating logic:
  • Not in thread → return (same as other modes)
  • Not involved → return
  • Other bot present → return (require @mention)

src/main.rs

• Initialize multibot_threads cache in Handler construction

docs/discord.md

• Add multibot-mentions to the allow_user_messages config table
• Add "Recommended: multibot-mentions mode" section under Multi-Bot Setup

Design decisions

Positive-only caching for other_bot_present: Like participation, a thread becoming multi-bot is irreversible — bot messages don't disappear. So we use the same positive-only cache pattern: once detected, cached for the session TTL. This avoids per-message API calls in active multi-bot threads.

Detection uses m.author.bot: Checks any bot author, not just trusted_bot_ids. This is intentional — even untrusted bots in a thread indicate a multi-bot scenario where @mention gating is desirable.

First message in new thread: No other bot messages exist yet → behaves as involved. Correct, since it is not yet a multi-bot thread.

[github-actions]

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Discord Discussion URL: https://discord.com/channels/...

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[thepagent]

LGTM — clean feature, well-designed middle ground between involved and mentions.

One nit: the PR description says "No caching of other_bot_present" but the code correctly caches it in multibot_threads. The code is right (bot posting is irreversible), the description just needs updating to match.

Everything else checks out:

• Config deserialization with -/_ normalization ✅
• Positive-only cache with TTL + eviction ✅
• Involved mode: no regression (bot_owns_thread still short-circuits) ✅
• MultibotMentions + bot_owns_thread: correctly fetches to check for other bots ✅
• Detection uses m.author.bot (any bot, not just trusted) ✅
• Docs updated ✅