feat: Anthropic API budget checker, fails open on error

tests/test_check_budget.py

@@ -0,0 +1,22 @@
+import sys
+sys.path.insert(0, 'bin')
+from unittest.mock import patch
+import pytest
+from check_budget import is_budget_ok, get_used_usd
+
+def test_budget_ok_when_used_less_than_limit():
+    with patch('check_budget.get_used_usd', return_value=2.0):
+        assert is_budget_ok(limit_usd=5.0) is True
+
+def test_budget_not_ok_when_used_exceeds_limit():
+    with patch('check_budget.get_used_usd', return_value=5.5):
+        assert is_budget_ok(limit_usd=5.0) is False
+
+def test_budget_ok_on_api_error():
+    """Fail open: if we can't check the budget, assume OK to avoid blocking research."""
+    with patch('check_budget.get_used_usd', side_effect=Exception("network error")):
+        assert is_budget_ok(limit_usd=5.0) is True
+
+def test_budget_exactly_at_limit_is_not_ok():
+    with patch('check_budget.get_used_usd', return_value=5.0):
+        assert is_budget_ok(limit_usd=5.0) is False