docs(readme): fix syntax errors in StoryAgent and test snippets

[u9g]

Summary

• Add missing comma after the instructions= f-string in the StoryAgent.__init__ super().__init__(...) call — without it, Python raises SyntaxError at the next keyword argument (llm=).
• Change async AgentSession(llm=llm) as sess: to async with AgentSession(llm=llm) as sess: in the testing snippet — the previous form is not valid Python.

Both snippets are copy-paste hazards: README fenced ```python blocks aren't executed in CI, so the errors went unnoticed.

Test plan

• Copy the updated StoryAgent snippet into a file and verify it parses with python -c "import ast; ast.parse(open('f.py').read())".
• Copy the updated test snippet and verify it parses.
• (Optional follow-up) Add a CI step that runs py_compile over fenced ```python blocks in README.md to prevent regressions.

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.