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>