* feat(slack): add download-file action for on-demand file attachment access
Adds a new `download-file` message tool action that allows the agent to
download Slack file attachments by file ID on demand. This is a prerequisite
for accessing images posted in thread history, where file attachments are
not automatically resolved.
Changes:
- Add `files` field to `SlackMessageSummary` type so file IDs are
visible in message read results
- Add `downloadSlackFile()` to fetch a file by ID via `files.info`
and resolve it through the existing `resolveSlackMedia()` pipeline
- Register `download-file` in `CHANNEL_MESSAGE_ACTION_NAMES`,
`MESSAGE_ACTION_TARGET_MODE`, and `listSlackMessageActions`
- Add `downloadFile` dispatch case in `handleSlackAction`
- Wire agent-facing `download-file` → internal `downloadFile` in
`handleSlackMessageAction`
Closes#24681
* style: fix formatting in slack-actions and actions
* test(slack): cover download-file action path
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Co-authored-by: Tak Hoffman <781889+Takhoffman@users.noreply.github.com>
