feat: add structured output support

Add support for Agent SDK structured outputs.

New input: json_schema - JSON schema for validated outputs
Auto-sets GitHub Action outputs for each field

Security:
- Reserved output protection (prevents shadowing)
- 1MB output size limits enforced
- Output key format validation
- Objects/arrays >1MB skipped (not truncated to invalid JSON)

Tests:
- 26 unit tests
- 5 integration tests
- 480 tests passing

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

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

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

action.yml

@@ -113,6 +113,10 @@ inputs:
     description: "Newline-separated list of Claude Code plugin marketplace Git URLs to install from (e.g., 'https://github.com/user/marketplace1.git\nhttps://github.com/user/marketplace2.git')"
     required: false
     default: ""
+  json_schema:
+    description: "JSON schema for structured output validation. When provided, Claude will return validated JSON matching this schema, and the action will automatically set GitHub Action outputs for each field."
+    required: false
+    default: ""
 
 outputs:
   execution_file:
@@ -174,6 +178,7 @@ runs:
         TRACK_PROGRESS: ${{ inputs.track_progress }}
         ADDITIONAL_PERMISSIONS: ${{ inputs.additional_permissions }}
         CLAUDE_ARGS: ${{ inputs.claude_args }}
+        JSON_SCHEMA: ${{ inputs.json_schema }}
         ALL_INPUTS: ${{ toJson(inputs) }}
 
     - name: Install Base Action Dependencies
@@ -228,6 +233,7 @@ runs:
         INPUT_SHOW_FULL_OUTPUT: ${{ inputs.show_full_output }}
         INPUT_PLUGINS: ${{ inputs.plugins }}
         INPUT_PLUGIN_MARKETPLACES: ${{ inputs.plugin_marketplaces }}
+        JSON_SCHEMA: ${{ inputs.json_schema }}
 
         # Model configuration
         GITHUB_TOKEN: ${{ steps.prepare.outputs.GITHUB_TOKEN }}