fix(messaging): attach Slack handoffs in source threads

[huntharo]

Summary

• I fixed Slack Agent handoffs so a request from a channel message attaches to a native Slack thread under that source message instead of reporting that child conversations are unsupported.
• I reuse Slack's preserved source timestamp as thread_ts; the first status delivery materializes the thread, while nested threads and sources without a message timestamp remain unsupported.
• I added regression coverage from an inbound app mention through managed-conversation discovery, binding creation, and threaded delivery.

Verification

• pnpm test — 4,743 passed, 3 skipped
• pnpm typecheck
• pnpm lint:eslint — 0 errors; existing warning baseline only
• pnpm lint:boundaries

Notes

• This uses the Slack adapter's existing chat.postMessage path and does not require an additional OAuth scope.