Write full file contents to the workspace tmp directory only.

Usage:
- `path` is resolved relative to the workspace `tmp/` directory.
- You may also pass a `tmp/...` path; it will still be constrained to the managed tmp directory.
- This tool creates a file or fully replaces an existing file under `tmp/`.
- Prefer `edit` when you only need a targeted change inside an existing tmp file.
- Avoid using this tool for partial updates; provide the complete desired file content.
- If the target path is a directory, the tool fails.
- The result includes a short diff summary of what changed.
