Root cause: PluginHookAgentContext in before_model_resolve only has
agentId, sessionKey, sessionId, workspaceDir, messageProvider.
senderId, channelId, input are NOT available in this hook phase.
The plugin was reading ctx.senderId (undefined) -> inHumanList=false
for ALL Discord sessions -> shouldUseNoReply=true -> all silenced.
Fix: use event.prompt which contains the full user message including
the 'Conversation info (untrusted metadata)' JSON block, and extract
sender_id from there. Same fix applied to before_prompt_build.
