d4b20f5295
Top-level channel messages from the same sender shared a bare channel debounce key, causing concurrent messages in different threads to merge into a single reply on the wrong thread. Now the debounce key includes the message timestamp for top-level messages, matching how the downstream session layer already scopes by canonicalThreadId. Extracted buildSlackDebounceKey() for testability. Closes #31935 Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>