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/README.md

@@ -25,5 +25,6 @@ Provide the path to the skill directory to audit when invoking.
 | `scripts/validate.sh` | Structural validator — checks name format, name matches directory, description length, line count, placeholder detection, script executable bit, and interactive-prompt detection |
 | `references/description-quality.md` | Spec-grounded rubric for description auditing — loaded when a finding is borderline |
 | `references/body-discipline.md` | Spec-grounded rubric for body discipline auditing — loaded when padding vs necessity is unclear |
+| `references/sources.md` | Provenance record — agentskills.io sources that informed this skill and which files each contributed to |
 | `tests/validate.bats` | Bats test suite for validate.sh |
 | `tests/README.md` | Setup instructions for bats-support and bats-assert test dependencies |