feat(skill-author): record research sources as skill provenance

Adds a sources provenance step to the skill creation workflow so the
chain from research output to skill content is traceable. Closes #4.

- New scaffold template `assets/templates/references/sources.md` mirroring
  the research skill's sources.md format (slug → URL, description,
  contributing files, status)
- `source_keys` commented-out optional field added to the SKILL.md
  template, mirroring how research topic files link back to sources
- New Step 5 in the creation workflow: populate references/sources.md
  from research input (attributing contributing skill files) or delete it
  if no research was provided; add source_keys to SKILL.md and any
  references/*.md files

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

plugins/kyberforge/skills/skill-author/README.md

@@ -40,6 +40,7 @@ If the destination is inside a plugin directory, read `references/deployment-mod
 | `assets/templates/README.md` | Annotated README template for the new skill |
 | `assets/templates/scripts/README.md` | Placeholder for bundled scripts |
 | `assets/templates/references/README.md` | Placeholder for reference docs |
+| `assets/templates/references/sources.md` | Sources provenance template for new skills |
 | `assets/templates/assets/README.md` | Placeholder for static assets |
 | `assets/templates/tests/README.md` | Placeholder for test files |
 | `tests/new-skill.bats` | Bats test suite for `scripts/new-skill.sh` |