miner-ui(hosted): first-run onboarding — connect GitHub, pick repos, first real discovery conversation

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Context

Part of the hosted AMS chat platform epic; blocked on the hosted-mode flag sub-issue and consuming tenant auth + hosted grounding. The bar mirrors the maintainer side: a contributor goes from "never heard of LoopOver" to a first real discovery conversation with zero human intervention — connect GitHub, pick repos, and the first chat shows actual claimable work.

Requirements

1. First-run flow: sign in with GitHub → select target repos → land in a first conversation that grounds on real discovery output ("here are the top claimable issues matching your profile across your selected repos", from the real discovery tools — no canned demo data).
2. Goal-spec respect from the first interaction: repo selection and discovery honor each target repo's .loopover-miner.yml (minerEnabled: false repos are excluded with the reason shown; wantedPaths/blockedPaths/claim caps flow through) — the onboarding path must not create a bypass around maintainer intent.
3. Defined empty/edge states, each reaching a useful next step: no selectable repos, all selected repos miner-disabled, discovery returns nothing, and Gittensor-registered vs unregistered users (registration is optional — the unregistered path is first-class, with the Gittensor-native extras presented as additive).
4. Safe defaults: onboarding configures nothing autonomous; running the miner unattended is a separate, explicit, post-onboarding action.
5. Funnel instrumentation per step (privacy-safe, PostHog), so contributor activation is measurable alongside the maintainer funnel.

Deliverables

• The first-run flow + guided first conversation, feature-flagged off by default.
• Tests: flow-state coverage of the four edge states, goal-spec-respect assertions (a minerEnabled: false repo can never be selected into discovery), safe-default assertions, funnel-event emission.

Expected outcome

The contributor half of "start using LoopOver like you'd start using Cursor": minutes from sign-in to seeing real work worth doing, registered on Gittensor or not.

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JSO-6358