Add knip for dead code detection, remove unused test utilities

[Copilot]

Integrated knip to identify unused code, dependencies, and exports. Removed 7 unused test helper functions discovered during initial analysis.

Changes

• Added knip dependency (5.82.1, exact version) with lint:knip npm script
• Created knip.json - entry point src/gh-widget-init.ts, ignores demo/dist/test files, excludes sass (peer dep of esbuild-sass-plugin)
• Removed unused test utilities:
  • setupCacheWithData(), resetCacheMock() from cache-mock.ts
  • createNotModifiedResponse() from fetch-mock.ts
  • createMockRepository(), createMockProfile() from mock-github-data.ts
  • setupTestDOM(), setupCommonTestHooks() from test-utils.ts

Analysis Result

$ npm run lint:knip
✂️ Excellent, Knip found no issues.

No production code modified. All changes isolated to test utilities.

Original prompt

Validate unused configs, code with knip. Apply cleanup.

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