feat(core): add provider-adapter-author skill

Converts a target repo's provider-specific instruction file (CLAUDE.md,
.cursor/rules, copilot-instructions.md, etc.) into a thin adapter over
AGENTS.md, mirroring this repo's own two-tier CLAUDE.md pattern
(ADR-0002/0003). Self-validates via a bundled deterministic script
(scripts/validate-adapter.sh) rather than a separate paired audit skill.

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

plugins/core/skills/provider-adapter-author/references/sources.md

@@ -0,0 +1,9 @@
+# Sources
+
+## adr-0002-0003-two-tier-claude-md
+
+- **URL:** (in-repo precedent — not an external source or plugin research corpus entry)
+- **Description:** This repo's own two-tier CLAUDE.md/AGENTS.md pattern: AGENTS.md is the provider-agnostic source of always-on rules; provider-specific files (CLAUDE.md) become thin adapters that import it (`@AGENTS.md` plus provider-specific additions). Grounds this skill's entire adapter-conversion design — the "thin adapter" shape, the `@`-import convention, and the size/duplication expectations enforced by `scripts/validate-adapter.sh`.
+- **Research doc:** docs/adr/0002-two-tier-claude-md.md, docs/adr/0003-agents-md-provider-agnostic-entry-point.md, providers/claude-code/CLAUDE.md (in-repo ADRs and a live example, not a plugin research corpus entry; referenced here since this skill's design is modeled directly on an existing implementation rather than external research)
+- **Contributing files:** SKILL.md
+- **Status:** `extracted`