Plugin should use vector search mode, not default hybrid

[bm-clawd]

Problem

The plugin's bm_search tool uses default search mode, which triggers hybrid RRF fusion. Due to basic-memory#577, hybrid fusion dilutes strong vector matches with weak FTS scores, producing unreliable relevance.

Evidence

Same query returns:

• Vector-only: 65% relevance ✅
• Hybrid (default): 3% relevance ❌

The plugin should use --vector (or --hybrid with a fixed RRF) to get meaningful semantic results.

Proposed Fix

Until basic-memory#577 is resolved:

1. Default to --vector search mode when semantic search is enabled
2. Fall back to FTS when semantic search is not available
3. Optionally: let the agent choose search mode per query

Longer Term

Once BM core fixes the RRF fusion, switch back to hybrid as default.

Related

• basic-memory#577 (hybrid RRF fusion bug)
• BUG: Semantic search not enabled — search relevance critically poor #16 (semantic search not working — now diagnosed as missing embeddings)