Update from bb (#126)

This pull request updates the `.github/workflows/struct-generate.yaml`
workflow to improve flexibility and security in how STRUCT files are
handled, and to ensure the workspace is clean before generating pull
requests. The most important changes are grouped below.

**Workflow input improvements:**

* Made the `struct_file` and `output_dir` inputs optional, and updated
the default value for `struct_file` to use a URI scheme
(`file://.struct.yaml`) for better compatibility.

**Environment and security enhancements:**

* Added the `GITHUB_TOKEN` environment variable to the STRUCT job,
allowing secure access to GitHub APIs during execution.

**Workspace cleanup before PR generation:**

* Added a step to remove the `custom-structures` directory before
generating a pull request, ensuring that no residual files affect the PR
creation process.

Author: httpdss