feat: Add contributor-facing LocalRetriever with sample INCF dataset fixtures for offline development

[QuantumByte-01]

Problem

PR #57 merged LocalRetriever but search() always returns [], making local dev useless — the agent always responds with "no results found" without GCP.

Solution

Add ~15 realistic INCF dataset fixtures in backend/fixtures/sample_datasets.json and load them in LocalRetriever.search() with basic keyword filtering.

Acceptance Criteria

• LocalRetriever.search() returns relevant fixture results
• Fixtures cover multiple modalities (fMRI, ephys, calcium imaging)
• README documents that local dev uses fixture data
• No new dependencies