#2414: fix(slack): route DM replies to original channel, not App Home

Fixes #2412 ## Problem When @mentioned in a DM between two other users (User A ↔ User B), the bot replies to App Home instead of the original DM. ## Root Cause `prepare.ts:440` sets `slackTo = user:${message.user}` for DMs. In `send.ts:81`, `user:X` targets trigger: ```typescript await client.conversations.open({ users: recipient.id }) ``` This opens a **new** bot↔user DM (App Home), not the original channel. ## Fix Always use `channel:${message.channel}` — the Slack API accepts channel IDs for all conversation types (channels, DMs, MPIMs). `lastRoute` tracking (line 538) intentionally still uses `user:X` for identity/session purposes. ## Testing - [x] Verified code path in `send.ts` confirms `user:` prefix triggers `conversations.open` - [x] Manual test: @mention bot in DM between two other users Co-Authored-By: Warp <agent@warp.dev> <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> This PR fixes Slack DM reply routing by always setting `To`/`OriginatingTo` to `channel:${message.channel}` in `src/slack/monitor/message-handler/prepare.ts`, preventing the downstream Slack sender from treating DMs as `user:` targets (which triggers `conversations.open` and creates a new bot↔user DM/App Home). Session/identity tracking via `lastRoute` remains keyed by `user:${message.user}` for DM contexts. <h3>Confidence Score: 5/5</h3> - This PR is safe to merge with minimal risk. - Change is localized to Slack inbound message preparation and aligns with Slack’s API expectations: replies are now always sent to the original channel ID, avoiding the unintended `conversations.open` path that created a new bot↔user DM. No broader behavior changes beyond DM routing are apparent. - src/slack/monitor/message-handler/prepare.ts <!-- greptile_other_comments_section --> _{(2/5) Greptile learns from your feedback when you react with thumbs up/down!} <!-- /greptile_comment -->