chore: add jscpd duplicate detection to check scripts

Integrate jscpd via a new 'check-duplicates' script and wire it into
both 'check' and 'check:ci' so duplicate code is caught in CI.

Author: kenryu42

package.json

@@ -16,12 +16,13 @@
     "build:types": "tsc --project tsconfig.build.json --emitDeclarationOnly --declaration",
     "build:schema": "bun run scripts/build-schema.ts",
     "clean": "rm -rf dist",
-    "check": "bun run lint && bun run typecheck && bun run knip && bun run sg:scan && AGENT=1 bun test --coverage",
-    "check:ci": "bun run lint:ci && bun run typecheck && bun run knip && bun run sg:scan && AGENT=1 bun test --coverage --coverage-reporter=lcov",
+    "check": "bun run lint && bun run typecheck && bun run knip && bun run check-duplicates && bun run sg:scan && AGENT=1 bun test --coverage",
+    "check:ci": "bun run lint:ci && bun run typecheck && bun run knip && bun run check-duplicates && bun run sg:scan && AGENT=1 bun test --coverage --coverage-reporter=lcov",
     "lint": "biome check --write",
     "lint:ci": "biome ci .",
     "typecheck": "tsc --noEmit",
     "knip": "knip --production",
+    "check-duplicates": "bunx jscpd src tests --exitCode 1 --reporters ai",
     "sg:scan": "ast-grep scan",
     "test": "bun test",
     "publish:dry-run": "bun run scripts/publish.ts --dry-run",