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/SKILL.md

@@ -0,0 +1,54 @@
+---
+name: provider-adapter-author
+description: >
+  Use when the user wants to convert a provider-specific AI instruction file
+  (CLAUDE.md, .cursor/rules/*.mdc, copilot-instructions.md, etc.) into a
+  thin adapter that defers to a repo's AGENTS.md — e.g. "reduce duplication
+  between CLAUDE.md and AGENTS.md", "make CLAUDE.md just import AGENTS.md"
+  — even if the pattern isn't named explicitly. Also invoke when
+  agentsmd-author detects an existing provider file overlapping with
+  AGENTS.md it just wrote. Detects redundant content in a provider file
+  relative to AGENTS.md and rewrites it down to a minimal reference (an
+  `@AGENTS.md`-style import where supported, or a text pointer otherwise)
+  plus genuinely provider-specific additions. Self-validates via a bundled
+  deterministic script before finishing. Do not use to write or audit
+  AGENTS.md itself — use agentsmd-author or agentsmd-audit.
+allowed-tools: Bash Read Edit Write
+metadata:
+  category: docs
+  source_keys:
+    - adr-0002-0003-two-tier-claude-md
+  version: "0.1.0"
+---
+
+## Gotchas
+
+- Not every provider supports cross-file imports. Claude Code does — a `CLAUDE.md` can consist of nothing but one or more `@path` lines (e.g. `@AGENTS.md`), with no other content required. Cursor's `.cursor/rules/*.mdc` and GitHub Copilot's `copilot-instructions.md` have no native import mechanism as of current tooling — for those, "thin" means a short text pointer to AGENTS.md plus only what that tool actually needs, not a literal import line. Pass `--no-import-syntax` to `scripts/validate-adapter.sh` for these providers.
+- This skill never creates or edits `AGENTS.md` itself. If the target repo has no `AGENTS.md` yet, stop and point the user to `agentsmd-author` first — there's nothing to adapt to.
+- Only strip content from the provider file that's genuinely redundant with AGENTS.md. Provider-specific material (IDE settings, tool-only syntax, model-specific instructions) stays — the goal is thin, not empty.
+- Works standalone or composed-into by `agentsmd-author` — behave identically either way; don't assume a caller skill exists.
+
+## Step 1 — Detect
+
+Look for known provider instruction files in the target repo: `CLAUDE.md` (repo root, and any deployed copies), `.cursor/rules/*.mdc`, `.github/copilot-instructions.md`, and similar tool-specific files. Confirm `AGENTS.md` exists at the repo root — if not, stop and tell the user to run `agentsmd-author` first.
+
+## Step 2 — Diff and rewrite
+
+Read the provider file and `AGENTS.md` side by side. Separate the provider file's content into two buckets: lines that restate what `AGENTS.md` already owns (universal rules, conventions, project overview) versus lines that are genuinely provider-specific (tool syntax, IDE behavior, model-specific instructions). Rewrite the provider file:
+
+- **Providers with import syntax** (Claude Code): replace the redundant bucket with an `@AGENTS.md` (or correct relative path) import line, keep the provider-specific bucket below it.
+- **Providers without import syntax** (Cursor, Copilot, etc.): replace the redundant bucket with a short pointer sentence mentioning `AGENTS.md`, keep the provider-specific bucket.
+
+## Step 3 — Self-validate
+
+Run the bundled check before finishing — this is the skill's own closeout gate; there is no separate paired audit skill for this concern:
+
+```bash
+bash scripts/validate-adapter.sh [--no-import-syntax] [--max-lines N] <adapter-file> <agents-md-file>
+```
+
+Fix any `FAIL` and re-run until it exits `0`.
+
+## Step 4 — Report
+
+State which file was converted, what was removed versus kept, and the validator's final result.