harden eval: prevent the calc tool from accessing globals and locals

By passing empty globals() and locals() to eval() we can prevent simple
malicious cases where the user gets the model to output something like

```<global variable/func> or "a".count("a")```
e.g.
```signal.raise_signal(9) or "a".count("a")``` which would kill the process.
or one could maybe get it to output secrets etc.

I think to make it 100% secure one would need to parse the AST and only execute secure nodes but this should make it much more robust.

nanochat/engine.py

@@ -37,7 +37,7 @@ def eval_with_timeout(formula, max_time=3):
         with timeout(max_time, formula):
             with warnings.catch_warnings():
                 warnings.simplefilter("ignore", SyntaxWarning)
-                return eval(formula)
+                return eval(formula, {"__builtins__": {}}, {})
     except Exception as e:
         signal.alarm(0)
         # print(f"Warning: Failed to eval {formula}, exception: {e}") # it's ok ignore wrong calculator usage