refactor(core): retrofit provider-adapter-author to the ADR-0020 contract

Description 833 -> 239 chars, body 370 -> 387 words, Gotchas 41% -> 20%.
The file-type enumeration moves to a new references/provider-matrix.md;
the composition note was already in README.md.

Three defects a clean-context audit found, all fixed:

The 'never edits AGENTS.md' prohibition had become a justification clause
on the false branch of a conditional, so the common path never read a
sentence binding it. That matters because the bundled validator's own
remediation text tells the agent to move content into AGENTS.md, so a
size FAIL actively invited the prohibited edit. Restored as a standing
imperative, plus a counter at the step where the trap fires.

A Gotcha asserted that validate-adapter.sh fails without
--no-import-syntax. The flag is a no-op -- both branches reduce to the
same expression. Reverted to an instruction; the script defect is #115.

The boundary clauses used pronouns to dodge the #110 regex, and 'Not
auditing it' resolved to CLAUDE.md as readily as to AGENTS.md -- routing
'audit my CLAUDE.md' to a skill whose own description declines it.

Refs #99

plugins/core/skills/provider-adapter-author/SKILL.md

@@ -1,36 +1,28 @@
 ---
 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.
+  Use when a provider file (CLAUDE.md, .cursor rules, copilot-instructions)
+  duplicating the repo's AGENTS.md should be cut to a thin adapter.
+  Not writing the AGENTS file -> `agentsmd-author`.
+  Not auditing the AGENTS file -> `agentsmd-audit`.
 allowed-tools: Bash Read Edit Write
 metadata:
   category: docs
   source_keys:
     - adr-0002-0003-two-tier-claude-md
-  version: "0.1.0"
+  version: "0.1.1"
 ---
 
 ## 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.
+- Assume a provider has no cross-file import mechanism until you have confirmed it has one. Claude Code is the exception, not the rule: a `CLAUDE.md` may consist of nothing but `@path` lines, while the same `@AGENTS.md` line in a Cursor rule or a Copilot instructions file is inert text no tool resolves. Pass `--no-import-syntax` to `scripts/validate-adapter.sh` for those providers.
 - 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.
+Find the provider instruction file to convert. Before searching, read `references/provider-matrix.md` — skip it only when the target is already a known root `CLAUDE.md`, which is the common case.
+
+Then confirm `AGENTS.md` exists at the repo root. If it does not, stop and tell the user to run `agentsmd-author` first — there is nothing to adapt to.
 
 ## Step 2 — Diff and rewrite
 
@@ -39,6 +31,10 @@ Read the provider file and `AGENTS.md` side by side. Separate 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.
 
+The provider file is the only file this skill ever writes. Never create or edit `AGENTS.md` — not in this step, not in any step, whatever the payoff looks like.
+
+Strip only what is genuinely redundant. Provider-specific material stays even when it is short — the goal is thin, not empty.
+
 ## 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:
@@ -47,7 +43,7 @@ Run the bundled check before finishing — this is the skill's own closeout gate
 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`.
+Fix any `FAIL` by editing the provider file, and re-run until it exits `0`. The size `FAIL` advises moving provider-agnostic content into `AGENTS.md`; disregard that half of its wording and delete the redundant lines instead.
 
 ## Step 4 — Report