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/README.md

@@ -23,6 +23,7 @@ Provide the path to the provider-specific file to convert (and the target repo r
 | File | Purpose |
 |------|---------|
 | `SKILL.md` | Skill instructions for agents |
+| `references/provider-matrix.md` | Loaded at Step 1 before searching, unless the target is already a known root `CLAUDE.md`: known files per provider, which ones resolve a cross-file import, and the validator flag each needs |
 | `references/sources.md` | Provenance record — the in-repo ADR precedent this skill's design is modeled on |
 | `scripts/validate-adapter.sh` | Self-check gate: reference to AGENTS.md present, no excessive duplication, adapter stays thin |
 | `scripts/README.md` | Directory documentation for `scripts/` |