fix: remove irregular whitespace from lint gate (#3518)

## Summary
- replace a zero-width space in the comment-assertion gate documentation
- restore the clean-main ESLint gate without changing scanner behavior

## Test plan
- `node scripts/check-no-comment-assertions-in-tests.mjs`
- `pnpm exec eslint scripts/check-no-comment-assertions-in-tests.mjs`
- `pnpm check:changesets`
- `pnpm lint`

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Documentation**
* Clarified the explanation for a narrowly scoped test-checking
exception.
  * No runtime behavior or user-facing functionality changed.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

scripts/check-no-comment-assertions-in-tests.mjs

@@ -29,8 +29,8 @@ const TEST_FILE_PATTERN = /\.test\.(?:ts|tsx|mts|cts|mjs|cjs|js|jsx)$/;
 /*
 An assertion matcher applied to a string carrying an FNXC stamp.
 
-DELIBERATELY NARROW: an earlier draft also flagged `/*` and `*​/` as comment delimiters and produced
-24 false positives and zero true ones - every hit was a path glob or a legitimate pattern
+DELIBERATELY NARROW: an earlier draft also flagged opening and closing block-comment delimiters and
+produced 24 false positives and zero true ones - every hit was a path glob or a legitimate pattern
 assertion, which a regex cannot tell apart from comment prose. Comment prose is not reliably distinguishable from
 data by regex, so this check enforces the one unambiguous, observed case and the standing rule in
 AGENTS.md covers the rest for human and agent reviewers.