v2.4.0: Score merging fix + orphan cleanup + temporal decay tuning

Critical Fixes

• Score merging: FTS5 and LIKE fallback scores now merged via max() — previously FTS5 would "claim" a chunk with a low BM25 score, preventing the LIKE path from scoring it higher. This caused precision issues like "Entry 499" ranking below random unrelated chunks.
• Orphan cleanup: memory-index.js now detects and removes entries for files deleted from disk. Previously, stale records accumulated and could pollute search results.

Improvements

• Temporal decay: Half-life increased from 30 → 90 days. Old memories retain meaningful weight (90d = 50% vs previous 12.5%).
• Search precision: Verified with stress test (500 entries) — exact match now consistently ranks #1.