feat: add _scaffold starter template

[kenjonespizza]

What this adds

A starter scaffold (_scaffold/) — the template you copy to create a new Sanity starter. Instead of building from scratch or adapting an existing starter, run cp -r _scaffold my-new-starter and start customizing.

What's in the box

• Sanity Studio v5 — sample post schema (title, slug, portable text), Presentation Tool wired to the frontend, Vision Tool, and auto-updating TypeGen
• Next.js 16 frontend — React 19, Tailwind v4 + typography plugin, next-sanity v12 with live previews, draft mode, and visual editing — all pre-connected to the Studio
• Sanity Functions — hello-world documentEventHandler with Blueprint registration and Rolldown bundler, ready to extend
• Shared config packages — @starter/eslint-config, @starter/tsconfig, @starter/sanity-types so every workspace shares lint rules, compiler settings, and generated types
• pnpm workspace with catalog: — single-source dependency versions across studio, frontend, and functions
• One-command bootstrap — pnpm bootstrap deploys the blueprint, deploys the schema, extracts + generates TypeGen types, and seeds 3 sample posts
• CI workflow — validates template structure, type-checks all workspaces, verifies function bundles build
• Agent context — AGENT.md + CLAUDE.md symlink, plus a skills placeholder for Claude Code

How to use it

cp -r _scaffold my-new-starter
cd my-new-starter
# Customize schema, frontend, functions, README, seed data...

See the updated CONTRIBUTING.md for the full 9-step checklist.

Docs changes

• CONTRIBUTING.md — "Adding a new starter" now starts with cp -r _scaffold and includes a step-by-step customization guide
• README.md — points to _scaffold/ for creating new starters
• _scaffold/README.md — framed as a template-for-templates, not a standalone app

Also in this PR

• Fixes pre-commit hook to run oxfmt per-starter so local .oxfmtrc.json configs are respected
• Adds _scaffold job to monorepo CI (format check, lint, template validation)

Test plan

• pnpm create sanity@latest --template sanity-labs/starters/_scaffold works end-to-end
• CI passes: format check, lint, and @sanity/template-validator
• Pre-commit hook correctly formats files using each starter's local config
• Studio loads at localhost:3333 with post schema and seed data
• Frontend renders post list at localhost:3000 and individual posts at /posts/:slug
• pnpm validate passes

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