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/SKILL.md

@@ -14,6 +14,12 @@ description: >
 allowed-tools: Bash Read
 metadata:
   category: factory
+  source_keys:
+    - agentskills-home
+    - agentskills-spec
+    - agentskills-best-practices
+    - agentskills-optimizing-descriptions
+    - agentskills-using-scripts
 ---
 
 ## Gotchas