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/assets/templates/SKILL.md

@@ -31,6 +31,13 @@ description: >
 # Optional. Arbitrary key-value map. Common keys: author, version, category.
 # No restrictions on keys or values.
 
+# source_keys:
+#   - source-slug-one
+#   - source-slug-two
+# Optional. Populated when this skill was built from /research output.
+# Lists slugs from references/sources.md that informed this file.
+# Also add source_keys to each references/*.md file that was informed by research.
+
 # allowed-tools: Bash Read Write
 # Optional (experimental — support varies by client).
 # Space-separated list of pre-approved tools.