feat: increase default gardenSearch limit from 8 to 16

[mdproctor]

Summary

Increase DEFAULT_LIMIT in SearchResource.java from 8 to 16. The current limit is the primary bottleneck for retrieval quality.

Motivation

Benchmark analysis (#33, #36) revealed:

• gardenSearch returns 8 results at 87% precision (~7 relevant per query)
• grep returns 20 results at 66% precision (~13 relevant per query)
• The 94% → 87% regression (three-leg → BGE-M3) is caused by relevant entries being pushed to rank 9-12 by ranking noise
• The "DOMAIN_ABSENCE" failure mode was a misdiagnosis — entries exist but are buried outside the top-8

Increasing to 16 should recapture most lost relevant entries while maintaining high precision.

Scope

1. Change DEFAULT_LIMIT from 8 to 16 in SearchResource.java
2. Consider adjusting OVERFETCH_MULTIPLIER (currently 2x) for adaptive extension
3. Update GardenMcpTools default limit description
4. Re-run benchmark at limit=16 to measure impact

Acceptance Criteria

• Default limit is 16
• Benchmark shows precision maintained at ≥80% with more total relevant entries
• MCP tool description updated