#22101: fix(slack): dedupe mentions by ts fallback for app_mention

## Summary - Fixes duplicated Slack channel replies when a channel message is delivered both via `message` and `app_mention` events. - Normalize inbound Slack message keys to `ts || event_ts` before dedupe and thread-resolution, so Slack app mention payloads that omit `ts` still map to the same message identity. - Preserve context IDs when possible by collecting message IDs from `ts || event_ts` for grouped dispatch payloads. - Add regression tests for both handler dedupe behavior and thread-resolution lookup fallback. ## Root Cause `createSlackMessageHandler` only passed `message.ts` into dedupe and thread-resolution logic. When `app_mention` events arrive without `ts` but with `event_ts`, the same inbound post can be treated as distinct after the `message` event path, creating two lane enqueues and two embedded-agent runs. ## Fix Details - `src/slack/monitor/message-handler.ts` - Deduplicate with `const messageTs = message.ts ?? message.event_ts` before `markMessageSeen`. - Pass normalized `ts` through thread resolver so `thread_ts` lookup works consistently. - Track Message IDs from `ts || event_ts` when batching events. - `src/slack/monitor/thread-resolution.ts` - Use `ts || event_ts` for thread resolution cache key and API lookup. - Keep verbose logs aligned to the normalized identifier. ## Testing - `pnpm vitest run src/slack/monitor/message-handler.test.ts src/slack/monitor/thread-resolution.test.ts` - `pnpm exec oxlint src/slack/monitor/message-handler.ts src/slack/monitor/thread-resolution.ts src/slack/monitor/message-handler.test.ts src/slack/monitor/thread-resolution.test.ts` - `pnpm exec oxfmt --check src/slack/monitor/message-handler.ts src/slack/monitor/thread-resolution.ts src/slack/monitor/message-handler.test.ts src/slack/monitor/thread-resolution.test.ts` ## Confidence Score: 5/5 - This change is a narrowly scoped data-id normalization bugfix with deterministic behavior and zero user-facing semantic drift. - It prevents dual processing by making both Slack event shapes map to the same dedupe identity and ensures thread lookup remains deterministic when `ts` is absent. - The logic is covered by direct regression tests at both the inbound-message-handler and thread-resolution layers. <!-- greptile_comment --> <h3>Greptile Summary</h3> Normalizes Slack message identifiers to use `ts ?? event_ts` instead of just `ts`, preventing duplicate processing when `app_mention` events arrive without `ts` but with `event_ts`. The fix ensures both event types map to the same dedupe identity and thread resolution cache key. - Applied normalization at dedupe check (message-handler.ts:115-120) - Applied normalization in message ID collection for batched events (message-handler.ts:87-89) - Applied normalization in thread resolution cache key and API lookup (thread-resolution.ts:81-104) - Added regression tests for both handler dedupe and thread-resolution fallback scenarios <h3>Confidence Score: 5/5</h3> - This PR is safe to merge with minimal risk - Narrowly scoped data normalization fix with deterministic behavior, covered by regression tests, and zero semantic drift in message processing logic - No files require special attention _{Last reviewed commit: 1bfd4fc} <!-- greptile_other_comments_section --> <!-- /greptile_comment -->