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

Records the upstream agentskills.io sources that informed skill-audit,
continuing the research → docs → skill provenance chain.

- New references/sources.md with 7 extracted sources attributed to skill files;
  agentskills-llms-txt demoted to discovery-only comment per skill-author precedent
- source_keys frontmatter added to SKILL.md (5 slugs), references/body-discipline.md
  (agentskills-spec, agentskills-best-practices), and references/description-quality.md
  (agentskills-spec, agentskills-optimizing-descriptions)

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

plugins/kyberforge/skills/skill-audit/references/description-quality.md

@@ -1,3 +1,9 @@
+---
+source_keys:
+  - agentskills-spec
+  - agentskills-optimizing-descriptions
+---
+
 # Description Quality Reference
 
 Source: agentskills.io — optimizing-descriptions