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/config/loader.py

@@ -6,15 +6,29 @@ from pathlib import Path
 from nanobot.config.schema import Config
 
 
+# Global variable to store current config path (for multi-instance support)
+_current_config_path: Path | None = None
+
+
+def set_config_path(path: Path) -> None:
+    """Set the current config path (used to derive data directory)."""
+    global _current_config_path
+    _current_config_path = path
+
+
 def get_config_path() -> Path:
-    """Get the default configuration file path."""
+    """Get the configuration file path."""
+    if _current_config_path:
+        return _current_config_path
     return Path.home() / ".nanobot" / "config.json"
 
 
 def get_data_dir() -> Path:
-    """Get the nanobot data directory."""
-    from nanobot.utils.helpers import get_data_path
-    return get_data_path()
+    """Get the nanobot data directory (derived from config path)."""
+    config_path = get_config_path()
+    # If config is ~/.nanobot-xxx/config.json, data dir is ~/.nanobot-xxx/
+    # If config is ~/.nanobot/config.json, data dir is ~/.nanobot/
+    return config_path.parent
 
 
 def load_config(config_path: Path | None = None) -> Config: