feat(engine): extract buildIssueRagQuery to gittensory-engine

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Found during a 2026-07-08 miner-roadmap audit. src/review/issue-rag-wire.ts::buildIssueRagQuery is pure, isolated, string-only query-building logic — already tagged as forward groundwork for the miner (#2320) and explicitly preserved as intentional (not dead code) in commit 39cc9583c's message.

This is scoped narrowly: only the pure query-builder function, not the retrieval backend itself. retrieveContext (src/review/rag.ts) is Vectorize/D1-bound and not portable to a local CLI without a hosted round-trip — that's tracked as a separate open product-design question, not part of this issue. This extraction is independent of and doesn't need to wait for that decision.

Deliverables

• Extract buildIssueRagQuery to packages/gittensory-engine/src/ (a new small module or an existing suitable one)
• src/review/issue-rag-wire.ts becomes a thin re-export shim, OR keep the function there and just re-export from gittensory-engine for the miner side to import — whichever avoids awkward circular imports, since issue-rag-wire.ts also wires into rag.ts
• Export from the package's public entrypoint
• Existing coverage for buildIssueRagQuery passes unmodified

References

• src/review/issue-rag-wire.ts (source, feat(miner-rag): issue-centric RAG query builder (issue title/body → embeddable query text) #2320)
• src/review/rag.ts::retrieveContext (the NOT-in-scope retrieval backend — see the separate RAG-access design-decision issue)
• Wave 2 tracker 🧩 Wave 2 — Analyze, Plan, Create & Deploy (HELD tracker) #2353, Phase 2 (the broader "wire issue-centric RAG retrieval into a scoped MCP tool" item this is one small piece of)

[linear-code]

JSO-3242