feat(core): add agentsmd-audit skill files

The previous commit only landed the research-folder rename — a multi-path
git add silently failed and left CONTEXT.md, ADR-0012, and the actual skill
files unstaged. This lands them: the agentsmd-audit skill itself (three
deterministic validators for secrets, structure, and drift against a target
repo's AGENTS.md), its bats test suite, provenance record, and the
CONTEXT.md/ADR entries documenting why this lives in core rather than
kyberforge.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

plugins/core/skills/agentsmd-audit/references/sources.md

@@ -0,0 +1,33 @@
+# Sources
+
+## agents-md-official
+
+- **URL:** https://agents.md/
+- **Description:** Official agents.md website — format spec, common-sections checklist, precedence rules (nearest-file-wins, no merge across files), monorepo nesting patterns
+- **Research doc:** plugins/core/docs/research/docs/agentsmd/sources.md
+- **Contributing files:** SKILL.md
+- **Status:** `extracted`
+
+## context7-websites-agents-md
+
+- **URL:** context7:/websites/agents_md
+- **Description:** Context7 index of the official agents.md website — overview, governance, cross-tool compatibility, configuration examples
+- **Research doc:** plugins/core/docs/research/docs/agentsmd/sources.md
+- **Contributing files:** SKILL.md
+- **Status:** `extracted`
+
+## context7-agentsmd-agents-md
+
+- **URL:** context7:/agentsmd/agents.md
+- **Description:** Context7 index of the agentsmd/agents.md repository — format spec, nested monorepo patterns, file structure examples
+- **Research doc:** plugins/core/docs/research/docs/agentsmd/sources.md
+- **Contributing files:** SKILL.md
+- **Status:** `extracted`
+
+## governance-secrets-hard-prohibition
+
+- **URL:** (org convention — not a plugin research corpus entry)
+- **Description:** Hard prohibition on placing secrets, API keys, tokens, or credentials in code, config, prompts, or any output. Grounds the secrets/credentials check in `scripts/validate-secrets.sh` and Step 1 of SKILL.md — AGENTS.md is committed content, so an embedded real secret is a hard-prohibition violation, not a style nit.
+- **Research doc:** core/instructions/governance.md (org convention file, not a plugin research corpus entry; content is inlined here since plugins must be self-contained and this file may not exist wherever the plugin is installed)
+- **Contributing files:** SKILL.md
+- **Status:** `extracted`