Add Nomic query and document embedding prefixes

[renezander030]

What changed

• Add opt-in Nomic search_query: and search_document: embedding prefixes.
• Add separate vector collection and metadata-path configuration for safe side-by-side reindexing.
• Make embedding truncation Unicode-safe.
• Add benchmark variants and any-of-N gold task support for A/B evaluation.

Why

Nomic retrieval expects search_document: for indexed documents and search_query: for queries. The adapter previously embedded both without task prefixes, so the recommended asymmetric retrieval mode was unavailable. Prefixes must be enabled with a separately reindexed collection.

Measured impact

On a private 25-question, 1,900-task benchmark:

• Semantic: hit@1 84% -> 88%, recall@5 92% -> 96%, MRR 0.88 -> 0.92.
• Hybrid/find: hit@1 84% -> 88%, recall@5 96% -> 100%, MRR 0.89 -> 0.93.

No private benchmark data is included.

Validation

• npm test: lint, PII, claims, 225 tests, and all proof suites passed.
• Focused adapter test suite: 8/8 passed.
• Synthetic side-by-side benchmark variant test passed, including any-of-N gold task IDs.