feat(pmf): advisory newcomer-PR auto-guide (Phase-1-lite, advisory only)

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

Goal

Lower the maintainer friction of accepting newcomers by auto-posting advisory guidance/review on first-time / newcomer PRs. This is a deliberately thin, advisory-only slice pulled early from the agent layer — it serves onboarding without any merge/close autonomy.

Context

• Gate + AI-review engine already post advisory panels (src/services/ai-review.ts, src/github/app.ts). First-time-contributor-aware gating is feat(github-app): first-time-contributor-aware gating #552.
• This is Phase-1-LITE: advisory only. The full write-action layer stays parked at feat(agent): maintainer write-actions layer (request-changes/approve/merge/close/label) #778 / Wave 2 (roadmap(agent phase 1): maintainer auto-maintain (cloud action layer) #769).

Requirements / Deliverables

• Detect first-time / newcomer authors (reuse feat(github-app): first-time-contributor-aware gating #552 logic).
• Auto-post a welcoming, specific advisory review (anti-slop + how-to-fix + what-makes-it-merge-worthy) — never blocking, never auto-merge.
• Per-repo opt-in; respects redaction + public-safe boundary.

Outcome

Maintainers of owned repos accept more newcomer PRs with less effort; newcomers get actionable guidance → higher funnel completion.

Depends on

#552; advisory-only (explicitly NOT #778 auto-actions).

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Closing as not planned (2026-07-01 issue audit): PMF newcomer-guide funnel retired; advisory newcomer guidance already covered by the live advisory review engine.