chore(review): remove dead deriveContributionLanes, superseded by onboarding-pack.ts's own contribution-lane model

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Context

src/signals/focus-manifest.ts:812-832 fully implements and documents deriveContributionLanes ("Derive public-safe... contribution lanes from a focus manifest"), with an exported type, but has zero callers outside its own test. Production contributor-guidance output instead flows through a separate, differently-shaped RepoPolicyContributionLane/RepoOnboardingContributionLane model in src/signals/onboarding-pack.ts — suggesting deriveContributionLanes is a superseded, orphaned implementation from before onboarding-pack.ts's model was built.

Requirements

• Confirm via a fresh repo-wide search that deriveContributionLanes and its exported type have zero real callers.
• Confirm onboarding-pack.ts's RepoPolicyContributionLane/RepoOnboardingContributionLane model genuinely covers the same use case deriveContributionLanes was built for (read both implementations' doc comments to confirm equivalent intent, not just similar naming).
• If confirmed superseded, remove deriveContributionLanes and its type from focus-manifest.ts, along with its dedicated test.
• If the two models actually serve different purposes (e.g. one is public-facing and the other isn't, in a way that matters), keep both but fix deriveContributionLanes's misleading lack of any caller by wiring it into whatever public-facing surface actually needs it — don't leave an ambiguous outcome.

Deliverables

• Either deriveContributionLanes removed (if confirmed redundant with onboarding-pack.ts's model), or wired into a real public-facing call site (if genuinely distinct).

Expected Outcome

No orphaned, functionally-superseded contribution-lane implementation remains alongside the one actually in production use.

Links & Resources

• src/signals/focus-manifest.ts:812-832
• src/signals/onboarding-pack.ts (the model actually in production use)

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JSO-4276