When replying to a Slack thread, files attached to the root message were
not being fetched. The existing `resolveSlackThreadStarter()` fetched the
root message text via `conversations.replies` but ignored the `files[]`
array in the response.
Changes:
- Add `files` to `SlackThreadStarter` type and extract from API response
- Download thread starter files when the reply message has no attachments
- Add verbose log for thread starter file hydration
Fixes issue where asking about a PDF in a thread reply would fail because
the model never received the file content from the root message.
