fix(core): close five ways validate-adapter.sh graded an adapter it had not read

The adapter check shipped green on files it should have failed, and failed files it
should have passed. Each defect is a residual of the fix that closes #115.

A fenced, indented or HTML-commented `@AGENTS.md` counted as an import, though
Claude Code resolves none of them — the adapter deferred to nothing and the gate
said so approvingly. Import matching now runs against a character mask that marks
fenced blocks and HTML comments inert, and applies CommonMark's four-space rule.
The mask is deliberately not applied to prose pointers, where four-space
indentation is ordinary list continuation.

The encoding fix reached only BOM-carrying UTF-16/32. BOM-less UTF-16LE/BE and
UTF-32LE are valid UTF-8, so they still produced the exact false diagnosis the fix
was written to remove: "no reference to AGENTS.md" on a file whose first line is
`@AGENTS.md`. A NUL-byte check is the complete signal. BOM stripping is no longer
positional, which also drops the mirror-image false FAILs on a doubled or mid-file
BOM.

`@NOTAGENTS.md`, `@zzzAGENTS.md` and `@docs/does/not/exist/AGENTS.md` all passed:
the pattern had no path-segment boundary and the target was never resolved on disk.
Both now hold, and a zero-byte or blank target is reported rather than credited.
Unresolved candidates print as `Near miss:` lines so the author sees why a line was
not counted.

`--no-import-syntax` still used substring matching, so `Do NOT read AGENTS.md; it
is obsolete.` passed as a pointer. That is #115's own defect surviving in the flag's
other mode. A mention must now carry a deference cue and must not be negated.

An unreadable file passed `isfile()`, raised, and exited 1 with a traceback and no
FAIL line — the one exit code no document covered, while Step 3 says to re-run
until it exits 0. It now exits 3 with a diagnostic, and the README states all four
codes and what to do about each instead of "exits non-zero on any failure".

Tests 18 to 42. Two of the new cases initially survived their own mutation and were
strengthened: `@NOTAGENTS.md` was being rejected by the disk check before the token
boundary ran, and stripping that boundary leaves the fragment `NOT`, which the
negation cue then rejects for an unrelated reason.

Refs: #115

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

@@ -30,8 +30,8 @@ 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 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.
+- **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 — nor one inside a code fence, an indented block, or an HTML comment, nor one whose path does not resolve to a real, non-empty file on disk.
+- **Providers without import syntax** (Cursor, Copilot, etc.): replace the redundant bucket with a short sentence pointing at `AGENTS.md` ("See AGENTS.md at the repo root for ..."), keep the provider-specific bucket. A bare or negated mention is not a pointer and will not be credited.
 
 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.
 
@@ -45,7 +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`. 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.
+Fix any `FAIL` by editing the provider file, and re-run until it exits `0`. Exits `2` and `3` are not `FAIL`s and nothing was graded under either, so neither is a reason to touch the adapter: `2` means the invocation or the input is wrong (a bad, missing, or extra argument, an unknown option, or a file that is not UTF-8), and `3` means a named file exists but could not be read.
 
 ## Step 4 — Report