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>

docs/usage.md

@@ -185,6 +185,74 @@ For a comprehensive guide on migrating from v0.x to v1.0, including step-by-step
       Focus on the changed files in this PR.
 ```
 
+## Structured Outputs
+
+Get validated JSON results from Claude that automatically become GitHub Action outputs. This enables building complex automation workflows where Claude analyzes data and subsequent steps use the results.
+
+### Basic Example
+
+```yaml
+- name: Detect flaky tests
+  id: analyze
+  uses: anthropics/claude-code-action@v1
+  with:
+    anthropic_api_key: ${{ secrets.ANTHROPIC_API_KEY }}
+    prompt: |
+      Check the CI logs and determine if this is a flaky test.
+      Return: is_flaky (boolean), confidence (0-1), summary (string)
+    claude_args: |
+      --json-schema '{"type":"object","properties":{"is_flaky":{"type":"boolean"},"confidence":{"type":"number"},"summary":{"type":"string"}},"required":["is_flaky"]}'
+
+- name: Retry if flaky
+  if: fromJSON(steps.analyze.outputs.structured_output).is_flaky == true
+  run: gh workflow run CI
+```
+
+### How It Works
+
+1. **Define Schema**: Provide a JSON schema via `--json-schema` flag in `claude_args`
+2. **Claude Executes**: Claude uses tools to complete your task
+3. **Validated Output**: Result is validated against your schema
+4. **JSON Output**: All fields are returned in a single `structured_output` JSON string
+
+### Accessing Structured Outputs
+
+All structured output fields are available in the `structured_output` output as a JSON string:
+
+**In GitHub Actions expressions:**
+
+```yaml
+if: fromJSON(steps.analyze.outputs.structured_output).is_flaky == true
+run: |
+  CONFIDENCE=${{ fromJSON(steps.analyze.outputs.structured_output).confidence }}
+```
+
+**In bash with jq:**
+
+```yaml
+- name: Process results
+  run: |
+    OUTPUT='${{ steps.analyze.outputs.structured_output }}'
+    IS_FLAKY=$(echo "$OUTPUT" | jq -r '.is_flaky')
+    SUMMARY=$(echo "$OUTPUT" | jq -r '.summary')
+```
+
+**Note**: Due to GitHub Actions limitations, composite actions cannot expose dynamic outputs. All fields are bundled in the single `structured_output` JSON string.
+
+### Complete Example
+
+See `examples/test-failure-analysis.yml` for a working example that:
+
+- Detects flaky test failures
+- Uses confidence thresholds in conditionals
+- Auto-retries workflows
+- Comments on PRs
+
+### Documentation
+
+For complete details on JSON Schema syntax and Agent SDK structured outputs:
+https://docs.claude.com/en/docs/agent-sdk/structured-outputs
+
 ## Ways to Tag @claude
 
 These examples show how to interact with Claude using comments in PRs and issues. By default, Claude will be triggered anytime you mention `@claude`, but you can customize the exact trigger phrase using the `trigger_phrase` input in the workflow.