feat(FN-3620): wire Hermes chat message tools and document mailbox behavior

Merges FN-3620, completing the Hermes chat mailbox integration with wired message tools, locked sender/recipient contract tests, and mailbox behavior documentation, plus a small CSS polish adding `focus-visible` styles and token radius to the render toggle. A changeset for FN-3710 (cluster task ID o

Fusion-Task-Id: FN-3620

packages/engine/src/index.ts

@@ -4,6 +4,8 @@ export {
   createTaskDocumentReadTool,
   createTaskDocumentWriteTool,
   createTaskLogTool,
+  createSendMessageTool,
+  createReadMessagesTool,
   taskCreateParams,
   taskDocumentReadParams,
   taskDocumentWriteParams,