ref(onboarding): Make onboarding session exits explicit

[jaydgoss]

TLDR

Clearing the selected platform in SCM onboarding no longer discards the connected repository and every other staged choice. The four exits that genuinely want a clean slate now say so explicitly, with no change to their behavior.

Details

setSelectedPlatform(undefined) was routed through removeOnboarding(), so a setter named after one field wiped the entire session. That coupling was invisible at every call site, and it made the setter unsafe for any state that should outlive a platform change — which is the blocker for the messaging destination VDY-140 stacks on top of this. The refactor lands on its own because it changes legacy onboarding behavior and deserves review on those terms, not buried in the experiment work.

OnboardingContextProvider now exposes resetOnboarding alongside the existing clearDerivedState, and setSelectedPlatform behaves like every sibling setter: it writes one field. The four full-flow exits that relied on the destructive overload call resetOnboarding directly — the global Skip Onboarding link in onboarding.tsx, back-from-select-platform and project deletion without state preservation in useBackActions, and the legacy cleanup in useConfigureSdk. Each of those is a 1-for-1 rename: behavior is unchanged, only the intent is now greppable.

Routing the exits through a named function rather than keeping the overload means the destructive call is greppable and each site states its intent. The alternative — leaving the setter destructive and adding a second non-destructive setter — preserves the trap for the next caller who reasonably assumes a field setter touches one field.

The one user-visible consequence is in scmPlatformFeaturesCore, whose clearable platform Select calls onPlatformChange(undefined). That control previously destroyed the SCM integration and repository selection as a side effect of clearing a platform; it now leaves them intact. Reachable when no platform was detected and the manual picker is showing.

New coverage in onboardingContext.spec.tsx pins both halves: clearing a field preserves the rest of the session, and resetOnboarding clears persisted state. The reset test seeds sessionStorage directly rather than using the initialValue prop, because useSessionStorage's removeItem restores in-memory state back to initialValue — a seeded prop would be restored rather than cleared, which the real provider never does since it passes no initialValue.

Two session-exit behavior changes that an earlier revision of this PR carried have moved to #120992, the messaging-route PR that actually requires them, so this PR is a pure refactor.

Refs VDY-140

[linear-code]

VDY-140