Add selfserve write-up

[aahiltn]

Summary

• Adds a one-page technical retrospective for GenerateNU/selfserve (spring2026 cohort) to projects/spring2026/selfserve.md.
• Cold read of the repo: commit history, PR history, CLAUDE.md, and source (Go handlers + TanStack Router web app + Expo mobile app).
• Covers the AI-assisted task-creation pipeline (Genkit/Gemini prompt design, deterministic enrichment lookups, ambiguity-as-warning handling, Temporal-wrapped durable retries), the append-only request-versioning model with a derived activity feed, the kanban board's cross-column optimistic cache patching, and Clerk-as-tenancy-boundary auth.
• Verified (after a sanity check) that the onboarding-not-wired-to-backend finding reflects in-flight work, not an abandoned feature: the backend has a real PUT /users/{id}/onboard endpoint, and PR #467 wiring the frontend to it is open but unmerged — same story for the Redis caching effort (PRs #280/#430, both open).

Test plan

• Verified contributor list against git shortlog + GitHub contributors API (bots excluded).
• Verified cohort guess (spring2026) against first/last commit dates.
• Checked projects/spring2026/ for a naming collision — none found.
• Spot-verified the onboarding-wiring and Redis-caching findings directly against source + open PR state rather than trusting the research pass at face value.
• N/A — documentation-only change, no code to run.

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