🧹 fix code duplication in extractor scripts

[savvides]

🎯 What: Removed duplicate extraction functions (detectPageType, extractContentFromDOM, extractPageContent, detectCourseContext) by moving them to extension/shared/extractor-core.js.
💡 Why: It improves maintainability by removing code duplication and ensuring both the side panel and the content script operate off the same single source of truth without relying on duplicate .js or .cjs files.
✅ Verification: Verified by ensuring the shared script is correctly configured in manifest.json and index.html. Mocked the globals in tests and confirmed the extension unit tests (./test/test-extension.sh) and full test suite (./test/smoke-test.sh) pass with full coverage of the extractor script behaviors. Also accounted for a tricky shim PATH limitation in testing by ensuring cut is safely mocked for pyenv.
✨ Result: A more maintainable architecture that adheres to Chrome Extension constraints without introducing a bundler.

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PR created automatically by Jules for task 8219125013309872649 started by @savvides

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