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/scripts/README.md

@@ -0,0 +1,9 @@
+# scripts/
+
+Deterministic self-check this skill shells out to instead of relying on LLM judgment for a mechanical check.
+
+| File | Purpose |
+|------|---------|
+| `validate-adapter.sh` | Checks a rewritten provider file (CLAUDE.md, etc.) has a reference to AGENTS.md, doesn't duplicate its content, and stays under a thin-file line threshold |
+
+Takes `<adapter-file> <agents-md-file>`, with optional `--no-import-syntax` and `--max-lines N` flags. Prints `FAIL` findings to stdout and exits non-zero on any failure.

plugins/core/skills/provider-adapter-author/scripts/validate-adapter.sh

@@ -0,0 +1,141 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+usage() {
+  cat <<EOF
+Usage: validate-adapter.sh [--no-import-syntax] [--max-lines N] <adapter-file> <agents-md-file>
+
+Self-check gate for provider-adapter-author. Checks that a rewritten
+provider-specific instruction file (CLAUDE.md, .cursor/rules/*.mdc,
+copilot-instructions.md, etc.) is actually a thin adapter over AGENTS.md,
+not a duplicate copy of it.
+
+Arguments:
+  adapter-file     Path to the provider-specific file to check.
+  agents-md-file   Path to the AGENTS.md file it should defer to.
+
+Options:
+  --no-import-syntax  The target provider has no native cross-file import
+                       mechanism. Accept a plain-text pointer mention of
+                       "AGENTS.md" instead of requiring an @import-style line.
+  --max-lines N        Max non-blank lines allowed in the adapter file before
+                       it's considered no longer "thin". Default: 60.
+  --help, -h           Show this help and exit 0.
+
+Exit codes:
+  0  Adapter file passes all checks
+  1  One or more checks failed (empty file, no reference to AGENTS.md,
+     excessive duplication, or file too long)
+EOF
+}
+
+NO_IMPORT_SYNTAX=0
+MAX_LINES=60
+ARGS=()
+
+while [[ $# -gt 0 ]]; do
+  case "$1" in
+    --help|-h)
+      usage
+      exit 0
+      ;;
+    --no-import-syntax)
+      NO_IMPORT_SYNTAX=1
+      shift
+      ;;
+    --max-lines)
+      MAX_LINES="${2:-}"
+      shift 2
+      ;;
+    *)
+      ARGS+=("$1")
+      shift
+      ;;
+  esac
+done
+
+if [[ ${#ARGS[@]} -lt 2 ]]; then
+  echo "Error: adapter-file and agents-md-file are required." >&2
+  echo "" >&2
+  usage >&2
+  exit 1
+fi
+
+python3 -u - "${ARGS[0]}" "${ARGS[1]}" "$NO_IMPORT_SYNTAX" "$MAX_LINES" <<'PYTHON'
+import sys
+import os
+import re
+
+adapter_path, agents_md_path, no_import_syntax, max_lines = sys.argv[1:5]
+no_import_syntax = no_import_syntax == "1"
+max_lines = int(max_lines)
+
+if not os.path.isfile(adapter_path):
+    print(f"Error: '{adapter_path}' is not a file.", file=sys.stderr)
+    sys.exit(1)
+if not os.path.isfile(agents_md_path):
+    print(f"Error: '{agents_md_path}' is not a file.", file=sys.stderr)
+    sys.exit(1)
+
+with open(adapter_path, encoding="utf-8", errors="replace") as f:
+    adapter_content = f.read()
+with open(agents_md_path, encoding="utf-8", errors="replace") as f:
+    agents_md_content = f.read()
+
+has_fail = False
+
+if not adapter_content.strip():
+    print(f"FAIL  Adapter file is empty — {adapter_path}")
+    print("      Why: An empty adapter carries no reference to AGENTS.md and no provider-specific content.")
+    print("      Fix: Add at least an import (or text pointer) to AGENTS.md.")
+    print()
+    sys.exit(1)
+
+IMPORT_RE = re.compile(r'(?m)^\s*@\S*AGENTS\.md\s*$')
+lines = adapter_content.splitlines()
+import_lines = [ln for ln in lines if IMPORT_RE.match(ln)]
+
+if no_import_syntax:
+    has_reference = "AGENTS.md" in adapter_content
+else:
+    has_reference = bool(import_lines) or "AGENTS.md" in adapter_content
+
+if not has_reference:
+    has_fail = True
+    print(f"FAIL  Adapter has no reference to AGENTS.md — {adapter_path}")
+    if no_import_syntax:
+        print("      Why: This provider has no import syntax, so the adapter must at least mention AGENTS.md as a text pointer.")
+        print("      Fix: Add a sentence like \"See AGENTS.md at the repo root for shared conventions.\"")
+    else:
+        print("      Why: A thin adapter must import AGENTS.md (e.g. `@AGENTS.md`) rather than silently omitting it.")
+        print("      Fix: Add an `@AGENTS.md` (or equivalent relative path) import line.")
+    print()
+
+# --- Duplication check ---
+non_import_lines = [ln for ln in lines if not IMPORT_RE.match(ln)]
+adapter_lines = [ln.strip() for ln in non_import_lines if ln.strip()]
+agents_lines = {ln.strip() for ln in agents_md_content.splitlines() if ln.strip()}
+
+if adapter_lines:
+    overlap = sum(1 for ln in adapter_lines if ln in agents_lines)
+    ratio = overlap / len(adapter_lines)
+    if ratio > 0.3:
+        has_fail = True
+        print(f"FAIL  Adapter duplicates AGENTS.md content — {adapter_path}")
+        print(f"      Why: {ratio:.0%} of the adapter's non-import lines already appear verbatim in AGENTS.md. A thin adapter should import shared content, not restate it.")
+        print("      Fix: Remove the duplicated lines and rely on the AGENTS.md import (or pointer) instead.")
+        print()
+
+# --- Size check ---
+non_blank_count = len([ln for ln in lines if ln.strip()])
+if non_blank_count > max_lines:
+    has_fail = True
+    print(f"FAIL  Adapter is not thin — {adapter_path}")
+    print(f"      Why: {non_blank_count} non-blank lines exceeds the {max_lines}-line threshold for a thin adapter.")
+    print("      Fix: Move provider-agnostic content into AGENTS.md; keep only genuinely provider-specific additions here.")
+    print()
+
+if has_fail:
+    sys.exit(1)
+sys.exit(0)
+PYTHON