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/sources.md

@@ -0,0 +1,52 @@
+# Sources
+
+<!-- agentskills.io/llms.txt was used for initial source discovery and is not listed below; it contributed no skill file content directly. -->
+
+## agentskills-home
+
+- **URL:** https://agentskills.io/home.md
+- **Description:** Agent Skills overview — what it is, why it exists, progressive disclosure model, ecosystem of 35+ implementing tools
+- **Contributing files:** SKILL.md
+- **Status:** `extracted`
+
+## agentskills-spec
+
+- **URL:** https://agentskills.io/specification.md
+- **Description:** Complete SKILL.md format specification — frontmatter fields, constraints, body content, optional directories, progressive disclosure levels, file references, validation
+- **Contributing files:** SKILL.md, references/body-discipline.md, references/description-quality.md
+- **Status:** `extracted`
+
+## agentskills-best-practices
+
+- **URL:** https://agentskills.io/skill-creation/best-practices.md
+- **Description:** Best practices for skill creators — starting from real expertise, spending context wisely, calibrating control, instruction patterns (gotchas, templates, checklists, validation loops)
+- **Contributing files:** SKILL.md, references/body-discipline.md
+- **Status:** `extracted`
+
+## agentskills-optimizing-descriptions
+
+- **URL:** https://agentskills.io/skill-creation/optimizing-descriptions.md
+- **Description:** How to systematically test and improve skill descriptions for triggering accuracy — eval queries, trigger rate testing, train/validation splits, optimization loop
+- **Contributing files:** SKILL.md, references/description-quality.md
+- **Status:** `extracted`
+
+## agentskills-evaluating-skills
+
+- **URL:** https://agentskills.io/skill-creation/evaluating-skills.md
+- **Description:** Eval-driven skill quality improvement — test case design, workspace structure, assertion writing, grading, benchmarking, human review, iteration loop
+- **Contributing files:** (none — eval workflow not directly informing audit dimensions)
+- **Status:** `extracted`
+
+## agentskills-using-scripts
+
+- **URL:** https://agentskills.io/skill-creation/using-scripts.md
+- **Description:** Using scripts in skills — one-off commands, self-contained scripts with inline dependencies, designing scripts for agentic use (no interactive prompts, --help, structured output, idempotency)
+- **Contributing files:** SKILL.md
+- **Status:** `extracted`
+
+## agentskills-quickstart
+
+- **URL:** https://agentskills.io/skill-creation/quickstart.md
+- **Description:** Step-by-step guide to creating a first skill (roll-dice example), how discovery/activation/execution work in practice
+- **Contributing files:** (none — creation guide not directly informing audit criteria)
+- **Status:** `extracted`