chore(skill-author): backfill sources provenance from agentskillsio research

Records the upstream agentskills.io sources that informed skill-author,
completing the research → docs → skill provenance chain introduced in
the previous commit.

- New references/sources.md with all 7 extracted agentskillsio sources,
  Contributing files attributed per-source to SKILL.md, references/deployment-modes.md,
  and references/scripts.md
- source_keys frontmatter added to SKILL.md (all 7 slugs), references/deployment-modes.md
  (agentskills-spec), and references/scripts.md (agentskills-using-scripts)

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

plugins/kyberforge/skills/skill-author/references/scripts.md

@@ -1,3 +1,8 @@
+---
+source_keys:
+  - agentskills-using-scripts
+---
+
 # Scripts Reference
 
 ## Package runners (no install required)