feat(config): add review.impact_map toggle (default off) for the impact map

[JSONbored]

Config slice for #1971: the per-repo default-OFF flag that gates impact-map computation + rendering, wired through FocusManifestReviewConfig with a global env kill-switch, byte-identical when unset.

Deliverables

• Add impactMap: boolean|null to FocusManifestReviewConfig + parse/normalize, default null
• isImpactMapEnabled(env) kill-switch helper (mirror isRagEnabled src/review/rag-wire.ts:27)
• Resolver AND-ing manifest toggle + env flag
• Tests: manifest/env combinations, absent defaults, both branches

References

• src/signals/focus-manifest.ts:124
• src/review/rag-wire.ts:27
• src/review/inline-comments.ts:21

Part of #1971.

---

size: XS · held for later release (gittensor:bug once activated)

[linear-code]

JSO-2285