feat(distribution): optional --no-vectors mode (pure-lexical)

[HumanBean17]

Tracks long-term option unlocked by #431 (fork B). Design ref: docs/superpowers/specs/active/2026-07-12-hybrid-bm25-rrf-design.md.

Why

#431 makes pure-lexical (BM25) quality measurable. If eval shows it is "good enough" for some environments, a --no-vectors mode would drop the torch / lancedb / sentence-transformers dependency monster — a materially lighter distribution.

Scope

• An optional distribution/install mode that skips the vector stack entirely and runs BM25-only search.
• Gated on feat(search): hybrid BM25 + vector ranking via RRF (promote lexical to first-class) #431's eval demonstrating acceptable pure-lexical recall/MRR on the target codebase shape.

Why deferred from #431

#431 is the prerequisite that makes this reachable; it is explicitly out of scope there.

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