🧹 [Code Health] Remove unused agent imports in search_agent.py

[MingfengHong]

🎯 What: The code health issue addressed
Removed unused imports LiteratureSearchAgent and WosSearchAgent from paperseek/search_agent.py. Updated paperseek/__init__.py to import these directly from paperseek_core.agent to maintain backwards compatibility in the public API.

💡 Why: How this improves maintainability
Removing unused imports is low-risk, improves code clarity, and reduces dead code.

✅ Verification: How you confirmed the change is safe
Ran ruff check and the full test suite python -m pytest tests/ which all passed, confirming that the public API remains unchanged and the codebase functions properly.

✨ Result: The improvement achieved
Improved code health in paperseek/search_agent.py while ensuring zero breakage to downstream code that imports from paperseek.

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PR created automatically by Jules for task 9943879089987859184 started by @MingfengHong

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