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/references/provider-matrix.md

@@ -0,0 +1,23 @@
+---
+source_keys:
+  - adr-0002-0003-two-tier-claude-md
+---
+
+# Known provider instruction files
+
+Which files to look for when detecting a provider-specific instruction file, whether each provider
+resolves a cross-file import, and what a thin adapter therefore looks like for it.
+
+| Provider | File(s) | Import syntax | Thin adapter shape | Validator flag |
+|---|---|---|---|---|
+| Claude Code | `CLAUDE.md` at the repo root, plus any deployed copies | Yes — `@path` lines, e.g. `@AGENTS.md` | One or more `@` import lines; no other content is required | none |
+| Cursor | `.cursor/rules/*.mdc` | No | A short sentence pointing at `AGENTS.md`, plus the rule's own frontmatter and provider-specific body | `--no-import-syntax` |
+| GitHub Copilot | `.github/copilot-instructions.md` | No | A short sentence pointing at `AGENTS.md`, plus Copilot-only instructions | `--no-import-syntax` |
+| Anything else | tool-specific instruction file at whatever path the tool documents | Assume no | Text pointer, as above | `--no-import-syntax` |
+
+A provider not listed here is not evidence it has an import mechanism. Confirm against that tool's
+own documentation before emitting an `@`-style line; an unresolved import reads as literal text and
+silently drops every rule the adapter was supposed to defer to.
+
+Detection is a search, not a lookup: a repo may hold more than one of these, and each one converts
+independently against the same `AGENTS.md`.

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

@@ -5,5 +5,5 @@
 - **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
+- **Contributing files:** SKILL.md, references/provider-matrix.md
 - **Status:** `extracted`