Improve README from template note to implementation guide

## Summary
The current README is still template-level. Expand it into an implementation-focused guide so consumers can quickly use and extend this action.

## Scope
- Add a clear project overview and intended use cases.
- Document action inputs/outputs and behavior.
- Provide at least one complete workflow usage example.
- Add local development instructions (install, codegen, lint, build, bundle, test).
- Add troubleshooting notes for common setup/runtime issues.

## Acceptance Criteria
- README includes quickstart and usage examples that work as-is.
- Inputs/outputs in README match action.yml.
- Development and validation commands are documented.
- Template wording is reduced in favor of actionable implementation guidance.