Add support for resources

This issue is tracking support for generating a skill with separate resources:

```
pdf-skill/
├── SKILL.md (main instructions)
├── FORMS.md (form-filling guide)
├── REFERENCE.md (detailed API reference)
```

The Gemini step should decide that it wants a separate resource, and a post-processing step would pull that chunk out into a separate markdown file.

Start by tracing the Gemini step and the post-processing step mentioned in the issue, including how generated skills are represented and where their output files are written. Determine how a separate resource should be identified and extracted. Done means a generated skill can include separate markdown resources such as FORMS.md and REFERENCE.md alongside SKILL.md.

Written by the indexing model from the issue text.