Downgrade Knip 6 to 5 for CI compatibility

Knip 6 uses oxc-parser which attempts a 6GB ArrayBuffer allocation
that fails on the CI runner (3.7GB RAM, no swap). This is a known
oxc allocator issue (oxc-project/oxc#20513) with no fix yet.
Revert to Knip 5 which uses TypeScript's parser. Also revert the
NODE_OPTIONS workaround since it's no longer needed.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

package.json

@@ -10,7 +10,7 @@
 		"@biomejs/biome": "2.4.8",
 		"@vitest/coverage-v8": "^4.1.0",
 		"jscpd": "^4.0.8",
-		"knip": "^6.0.2",
+		"knip": "^5.88.1",
 		"lefthook": "^2.1.4",
 		"oxlint": "^1.56.0",
 		"oxlint-tsgolint": "^0.17.1",