#13655: acceptAdd nixpacks configuration for Python and Node.js

<!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> This PR adds a nixpacks configuration file for deploying OpenClaw to platforms like Railway, but the configuration is fundamentally incorrect. It treats OpenClaw as a Python project when it's actually a Node.js/TypeScript application. **Critical issues:** - References non-existent `requirements.txt` and `openclaw.app` Python module - Includes Python 3.11 in dependencies despite OpenClaw being a Node.js project - Incorrect start command (`python3 -m openclaw.app` instead of `node openclaw.mjs gateway --allow-unconfigured`) - Missing required build steps (`pnpm build` and `pnpm ui:build`) - Uses `--no-frozen-lockfile` instead of `--frozen-lockfile` (violates project conventions in AGENTS.md) The configuration needs to be completely rewritten to match the existing Dockerfile pattern, which correctly builds and runs OpenClaw as a Node.js application. <h3>Confidence Score: 0/5</h3> - This PR cannot be merged as-is — it would break deployment completely - The configuration is based on a fundamental misunderstanding of the project. It attempts to run OpenClaw as a Python application when it's actually a Node.js/TypeScript project. The start command references a non-existent Python module, install steps reference a non-existent `requirements.txt`, and critical build steps are missing. Deploying this would result in immediate runtime failures. - nixpacks.toml requires complete rewrite based on the Dockerfile pattern <!-- greptile_other_comments_section --> _{(2/5) Greptile learns from your feedback when you react with thumbs up/down!} <!-- /greptile_comment -->