feat(slack): add download-file action for on-demand file attachment access (#24723)

* 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

---------

Co-authored-by: Tak Hoffman <781889+Takhoffman@users.noreply.github.com>

src/channels/plugins/message-action-names.ts

@@ -50,6 +50,7 @@ export const CHANNEL_MESSAGE_ACTION_NAMES = [
   "kick",
   "ban",
   "set-presence",
+  "download-file",
 ] as const;
 
 export type ChannelMessageActionName = (typeof CHANNEL_MESSAGE_ACTION_NAMES)[number];