feat(miner-foundation): extract scoring preview/model modules into gittensory-engine

[JSONbored]

Move src/scoring/preview.ts, src/scoring/model.ts, and src/scoring/pending-pr-scenarios.ts into packages/gittensory-engine/src/scoring/ (as a subdirectory, since these are a cohesive group rather than standalone signals). These give the miner the same score-preview machinery the existing gittensory_preview_local_pr_score MCP tool already calls server-side, so a miner can preview its own PR score fully locally without a network round-trip.

Deliverables

• packages/gittensory-engine/src/scoring/preview.ts, packages/gittensory-engine/src/scoring/model.ts, packages/gittensory-engine/src/scoring/pending-pr-scenarios.ts — verbatim ports preserving the three-file structure.
• src/scoring/preview.ts, src/scoring/model.ts, src/scoring/pending-pr-scenarios.ts become thin re-export shims from @jsonbored/gittensory-engine.
• Existing scoring test suites pass unmodified against the shims.
• packages/gittensory-engine/src/index.ts re-exports the new scoring/ subpath (e.g. export * as scoring from "./scoring/preview" or equivalent — pick whichever barrel shape keeps import ergonomics closest to the existing src/scoring/* call sites).

References

• src/scoring/preview.ts, src/scoring/model.ts, src/scoring/pending-pr-scenarios.ts — the three files to port.
• packages/gittensory-engine/ — created by the package-skeleton issue (must land first).
• packages/gittensory-mcp/bin/gittensory-mcp.js (gittensory_preview_local_pr_score tool, ~line 448) — the existing server-side consumer this extraction gives a local-first equivalent for.

[linear-code]

JSO-2379