Discord's _fetch_channel_context backfills recent channel/thread activity
(from any member who can post there, not just the allowlisted user) into
the agent's context with no sender-trust distinction. Slack's equivalent
_fetch_thread_context was fixed to prefix non-allowlisted senders with
[unverified] and add LLM guidance not to act on their content, mitigating
indirect prompt injection from third parties in shared channels/threads.
Port the same mechanism to Discord using the already-wired
_is_sender_authorized/set_authorization_check plumbing.
8e94e8f882