feat: multi-instance support with --config parameter

Add support for running multiple nanobot instances with complete isolation:

- Add --config parameter to gateway command for custom config file path
- Implement set_config_path() in config/loader.py for dynamic config path
- Derive data directory from config file location (e.g., ~/.nanobot-xxx/)
- Update get_data_path() to use unified data directory from config loader
- Ensure cron jobs use instance-specific data directory

This enables running multiple isolated nanobot instances by specifying
different config files, with each instance maintaining separate:
- Configuration files
- Workspace (memory, sessions, skills)
- Cron jobs
- Logs and media

Example usage:
  nanobot gateway --config ~/.nanobot-instance2/config.json --port 18791

nanobot/utils/helpers.py

@@ -12,8 +12,9 @@ def ensure_dir(path: Path) -> Path:
 
 
 def get_data_path() -> Path:
-    """~/.nanobot data directory."""
-    return ensure_dir(Path.home() / ".nanobot")
+    """Get nanobot data directory (derived from config path)."""
+    from nanobot.config.loader import get_data_dir
+    return ensure_dir(get_data_dir())
 
 
 def get_workspace_path(workspace: str | None = None) -> Path: