feat: support ${VAR} env var interpolation in config secrets

Allow config.json to reference environment variables via ${VAR_NAME}
syntax. Variables are resolved at runtime by resolve_config_env_vars(),
keeping the raw templates in the Pydantic model so save_config()
preserves them. This lets secrets live in a separate env file
(e.g. loaded by systemd EnvironmentFile=) instead of plain text
in config.json.

nanobot/cli/commands.py

@@ -453,7 +453,7 @@ def _make_provider(config: Config):
 
 def _load_runtime_config(config: str | None = None, workspace: str | None = None) -> Config:
     """Load config and optionally override the active workspace."""
-    from nanobot.config.loader import load_config, set_config_path
+    from nanobot.config.loader import load_config, resolve_config_env_vars, set_config_path
 
     config_path = None
     if config:
@@ -464,7 +464,11 @@ def _load_runtime_config(config: str | None = None, workspace: str | None = None
         set_config_path(config_path)
         console.print(f"[dim]Using config: {config_path}[/dim]")
 
-    loaded = load_config(config_path)
+    try:
+        loaded = resolve_config_env_vars(load_config(config_path))
+    except ValueError as e:
+        console.print(f"[red]Error: {e}[/red]")
+        raise typer.Exit(1)
     _warn_deprecated_config_keys(config_path)
     if workspace:
         loaded.agents.defaults.workspace = workspace