Write full file contents to the workspace `tmp/` directory only.

Usage:
- `path` is resolved relative to the workspace `tmp/` directory.
- This tool creates a file or fully replaces an existing file under `tmp/`.
- Absolute paths and paths that escape `tmp/` are rejected.
- Prefer this tool for temporary reports, logs, and intermediate artifacts.
- If the target path is a directory, the tool fails.
- The result includes a short diff summary of what changed.
