Add chaining vignette and complete API reference

[JamesHWade]

Summary

• Add chaining vignette with run() and run_dataset() examples.
• Add ReAct module reference page and include it in pkgdown.
• Ensure API reference covers all exports (add Signature, ensemble, %>>%, and tidymodels helpers).

Testing

• Not run (docs-only changes).

[claude]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.