feat: add structured output support via --json-schema argument (#687)

* feat: add structured output support

Add support for Agent SDK structured outputs.

New input: json_schema
Output: structured_output (JSON string)
Access: fromJSON(steps.id.outputs.structured_output).field

Docs: https://docs.claude.com/en/docs/agent-sdk/structured-outputs

* rm unused

* refactor: simplify structured outputs to use claude_args

Remove json_schema input in favor of passing --json-schema flag directly
in claude_args. This simplifies the interface by treating structured outputs
like other CLI flags (--model, --max-turns, etc.) instead of as a special
input that gets injected.

Users now specify: claude_args: '--json-schema {...}'
Instead of separate: json_schema: {...}

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

* chore: remove unused json-schema util and revert version

- Remove src/utils/json-schema.ts (no longer used after refactor)
- Revert Claude Code version from 2.0.45 back to 2.0.42

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

---------

Co-authored-by: Claude <noreply@anthropic.com>

README.md

@@ -13,6 +13,7 @@ A general-purpose [Claude Code](https://claude.ai/code) action for GitHub PRs an
 - 💬 **PR/Issue Integration**: Works seamlessly with GitHub comments and PR reviews
 - 🛠️ **Flexible Tool Access**: Access to GitHub APIs and file operations (additional tools can be enabled via configuration)
 - 📋 **Progress Tracking**: Visual progress indicators with checkboxes that dynamically update as Claude completes tasks
+- 📊 **Structured Outputs**: Get validated JSON results that automatically become GitHub Action outputs for complex automations
 - 🏃 **Runs on Your Infrastructure**: The action executes entirely on your own GitHub runner (Anthropic API calls go to your chosen provider)
 - ⚙️ **Simplified Configuration**: Unified `prompt` and `claude_args` inputs provide clean, powerful configuration aligned with Claude Code SDK