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/references/sources.md

@@ -0,0 +1,13 @@
+# Sources
+
+<!-- Populated at Step 5 of skill authoring, after all skill files are written.
+     For each research source with status `extracted`, record which skill files
+     it contributed to under Contributing files.
+     Delete this file if no research sources were provided as input. -->
+
+## FILL IN: source-slug
+
+- **URL:** FILL IN
+- **Description:** FILL IN
+- **Contributing files:** FILL IN: list skill files this source informed (e.g. SKILL.md, references/foo.md)
+- **Status:** `extracted`