feat(sources): add Semantic Scholar adapter with citation context

[foundatron]

Problem Statement

The Semantic Scholar adapter needs API key support (for higher rate limits), citation count filtering, and the ability to fetch related papers through citation graphs.

Proposed Change

Enhance the Semantic Scholar adapter with production features.

Files to Modify

• tentacle/sources/semantic_scholar.py - Add API key header (S2_API_KEY env var), citation count minimum filter, retry logic, date range filtering
• tentacle/config.py - Add s2_api_key config field
• tests/test_sources.py - Add tests for API key header, filtering, error handling

Acceptance Criteria

• API key passed via x-api-key header when configured
• Minimum citation count filter (configurable, default 0)
• Date range filtering (publicationDate parameter)
• Retry on HTTP 429 with Retry-After header respect
• Tests cover API key injection, empty results, API errors

Design Notes

Semantic Scholar free tier: 100 requests/5 min. With API key: 1 request/second. The publicationDate filter uses YYYY-MM-DD:YYYY-MM-DD format.

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