fix(core): strip a BOM before the adapter import check, and split usage exits

Narrowing has_reference to bool(import_lines) meant a UTF-8 BOM hid the import
line, since the BOM is not \s: a CLAUDE.md whose first line is @AGENTS.md failed
with 'no reference to AGENTS.md' and was told to add the line already in front of
it. Decoding is now strict too, so a non-UTF-8 adapter gets an encoding
diagnostic instead of being mangled and then graded on the mangling.

Usage errors move to exit 2. They shared exit 1 with real findings, while the
skill tells the agent to fix any non-zero exit by editing the provider file.

The whole-line import rule is kept deliberately -- accepting an inline @AGENTS.md
would also accept one inside backticks, which is the silent-drop failure the
validator exists to catch -- and the message now says so.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EJJrm5YmacbwMdzZpXcoti

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

@@ -18,6 +18,8 @@ metadata:
 
 - 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; do not assume a caller skill exists. Detect the provider file, confirm `AGENTS.md`, and run the closeout validator yourself in both cases (`references/provider-matrix.md`).
+
 ## Step 1 — Detect
 
 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.
@@ -28,7 +30,7 @@ Then confirm `AGENTS.md` exists at the repo root. If it does not, stop and tell
 
 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 with import syntax** (Claude Code): replace the redundant bucket with an `@AGENTS.md` (or correct relative path) import on a line of its own, keep the provider-specific bucket below it. An import folded into a sentence is not the thin-adapter shape and `scripts/validate-adapter.sh` will not credit 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.
@@ -43,7 +45,7 @@ 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` by editing the provider file, and re-run until it exits `0`.
+Fix any `FAIL` by editing the provider file, and re-run until it exits `0`. Exit `2` is not a `FAIL`: it means the invocation or the input is wrong — a bad or missing argument, or a file that is not UTF-8 — so fix that, not the adapter.
 
 ## Step 4 — Report