feat(pmf): the conversion hook — 'this would score ~X on gittensor, register in 60s'

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Parent epic: PMF onboarding funnel (link below). Milestone M0.

Goal

The single most important PMF instrument: at the moment a non-registered contributor produces a quality PR, show them what it would be worth on gittensor and a frictionless register-and-claim path.

Context

• Scoring/score-preview exists (src/scoring/preview.ts) but is gated to registered contributors. The hook needs a public-safe, anon-visible 'estimated value + register CTA' that respects the redaction boundary (no raw scores/wallets — a band/range + CTA).
• Surfaces: PR panel comment, extension overlay (feat(extension): rebuild as dual-audience overlay (WXT + React) #534), CLI/MCP output.

Requirements / Deliverables

• Public-safe 'estimated gittensor value' band for a merged-quality PR (range, not raw score; passes redaction).
• Register-and-claim CTA wired into: PR panel comment, browser extension, CLI/MCP completion.
• Attribution so a post-registration claim links back to the pre-registration contribution.
• Measured as a funnel event (see funnel instrumentation issue).

Outcome (measurable)

% of non-registered quality-PR authors who see the hook and register — the core PMF conversion metric.

Depends on

P1 (no-account mode), funnel instrumentation, #534 extension.

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Closing as not planned (2026-07-01 issue audit): Conversion hook (anon estimate + register CTA) is the non-registered->gittensor growth instrument retired by the pivot.