Remove ~900 lines of dead code across agent, backend, and frontend (#318)

Add CI for tests and Ruff (#217)

Minimize S2 API calls: drop paper_details enrichment, add response cache, rate limit only with API key

Add Semantic Scholar integration: citation graph, filtered search, snippet search, recommendations

fix: buffer websocket events during disconnects and replay on reconnect

fix: use correct HF API filter param and improve integration tests

feat: add hf_papers tool for paper discovery and reading