Feature Request: Add --max-turns to claude_args documentation

---
name: Feature request
about: Suggest an idea for claude-code-action
title: ''
labels: enhancement, documentation, p3
---

## Is your feature request related to a problem?
The `claude_args` input supports `--max-turns` but this is not documented in README.md or docs/usage.md.

## Proposed Solution
Add `--max-turns` to the documented claude_args examples in docs/usage.md.

## Additional Context
This is a documentation-only change. No code changes required.

LABELS: enhancement, documentation, p3

Start with the documented claude_args examples in docs/usage.md and compare them with the README.md mention. Add --max-turns to the relevant documentation examples, then verify that the documented input matches the supported claude_args options and that both referenced docs are consistent.

索引モデルが issue の本文から書いたものです。