Discord threads use their own channel IDs, so allowChannels was blocking
thread replies unless each thread ID was listed explicitly.
- Include the thread parent channel ID as an allowlist candidate
- Enforce allow_channels on slash commands (previously bypassed)
- Show parent channel ID in runtime context, reply to the thread
- Fix subagent cancel key via effective_key propagation
- Detect bot mentions via raw_mentions and reply-to-bot references
- Cache seen thread channels for outbound delivery
- Ignore system messages that become empty prompts
