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/references/provider-matrix.md

@@ -21,3 +21,12 @@ 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`.
+
+## Standalone and composed runs behave identically
+
+This skill is reached two ways: invoked directly by a user, and composed into by `agentsmd-author`
+once it has written or updated the repo's `AGENTS.md`. Behave identically either way — do not
+assume a caller skill exists. Detect the provider file yourself, confirm `AGENTS.md` yourself, and
+run the closeout validator yourself, rather than treating any step as already done by the caller or
+as something the caller will do afterwards. There is no handshake to rely on and no state passed
+in beyond the file paths.