build(deps): update openai-agents requirement to >=0.12,<0.18 (#78)

Manual replay of Dependabot PR #74. Dependabot's rebase produced
a conflict against the just-merged scanner 3.10 fix (#77), which
also touched engine/pyproject.toml.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: study8677

engine/pyproject.toml

@@ -9,7 +9,7 @@ dependencies = [
     "python-dotenv>=1.0,<2",
     "requests>=2.33.0,<3",
     "mcp[cli]>=1.0.0,<2",
-    "openai-agents[litellm]>=0.12,<0.15",
+    "openai-agents[litellm]>=0.12,<0.18",
     "tomli>=2.0,<3; python_version < \"3.11\"",
 ]