Write full file contents to the workspace.

Usage:
- `path` is resolved relative to the workspace root.
- This tool creates a file or fully replaces an existing file.
- Prefer `edit` when you only need a targeted change inside an existing 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.
