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.

hermes_cli/commands.py

@@ -26,6 +26,7 @@ COMMANDS_BY_CATEGORY = {
         "/title": "Set a title for the current session (usage: /title My Session Name)",
         "/compress": "Manually compress conversation context (flush memories + summarize)",
         "/rollback": "List or restore filesystem checkpoints (usage: /rollback [number])",
+        "/background": "Run a prompt in the background (usage: /background <prompt>)",
     },
     "Configuration": {
         "/config": "Show current configuration",