feat: add /background command to gateway and CLI commands registry

Add /background <prompt> to the gateway, allowing users on Telegram,
Discord, Slack, etc. to fire off a prompt in a separate agent session.
The result is delivered back to the same chat when done, without
modifying the active conversation history.

Implementation:
- _handle_background_command: validates input, spawns asyncio task
- _run_background_task: creates AIAgent in executor thread, delivers
  result (text, images, media files) back via the platform adapter
- Inherits model, toolsets, provider routing from gateway config
- Error handling with user-visible failure messages

Also adds /background to hermes_cli/commands.py registry so it
appears in /help and autocomplete.

Tests: 15 new tests covering usage, task creation, uniqueness,
multi-platform, error paths, and help/autocomplete integration.

tests/hermes_cli/test_commands.py

@@ -12,7 +12,7 @@ EXPECTED_COMMANDS = {
     "/personality", "/clear", "/history", "/new", "/reset", "/retry",
     "/undo", "/save", "/config", "/cron", "/skills", "/platforms",
     "/verbose", "/compress", "/title", "/usage", "/insights", "/paste",
-    "/reload-mcp", "/rollback", "/skin", "/quit",
+    "/reload-mcp", "/rollback", "/background", "/skin", "/quit",
 }