fix: workaround GitHub Actions composite action output limitation

GitHub Actions composite actions cannot have dynamic outputs - all outputs
must be explicitly declared in action.yml. This is a known limitation.

Changes:
- Add structured_output JSON output to base-action/action.yml
  (contains all structured fields as single JSON string)
- Update run-claude.ts to set structured_output output
- Update tests to parse structured_output JSON with jq
- Add structured_output to RESERVED_OUTPUTS list

Users can now access structured outputs via:
  steps.<id>.outputs.structured_output | jq '.field_name'

Or in GitHub Actions expressions:
  fromJSON(steps.<id>.outputs.structured_output).field_name

Individual field outputs are still set for direct usage contexts,
but only the structured_output JSON is accessible via composite action.

Fixes #683 test failures

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

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

base-action/action.yml

@@ -92,6 +92,9 @@ outputs:
   execution_file:
     description: "Path to the JSON file containing Claude Code execution log"
     value: ${{ steps.run_claude.outputs.execution_file }}
+  structured_output:
+    description: "JSON string containing all structured output fields (use fromJSON() or jq to parse)"
+    value: ${{ steps.run_claude.outputs.structured_output }}
 
 runs:
   using: "composite"