* feat(onboarding): hide agent chat step; subscribe after intro steps
Remove the hosted-only strategy-chat diversion from the onboarding
sequence. After the three intro questions, hosted users now hit the
subscribe paywall directly, then return to the GSC and MCP connect
steps. The chat route and components stay in place but unlinked, to be
revisited later. Preserve the post-payment 'You're in!' interstitial by
carrying checkout=success through validateSearch.
* fix(onboarding): set checkout=success from subscribe route, not speculatively
The previous redirect baked checkout=success into the onboarding return
URL at the point needsSubscription is true — i.e. before the user had
paid. It only worked because the subscribe route gates its redirect on
actual access. Move the marker to the subscribe route's redirect-to-app
path, where checkoutCompleted reflects a real returned-from-Stripe
payment, so the 'You're in!' screen can never show pre-payment.
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