feat(create-meith): scaffold plugin and theme workspaces from the worked examples

[jouwdan]

Part of MEI-115 (deliverable 1 of 4: scaffolds). The remaining deliverables — packed-kit smoke, authoring tutorial, and the approved first-cohort plugins — follow as separate PRs, per the ticket.

What this adds

npx create-meith --plugin <name> and npx create-meith --theme <name> write standalone extension workspaces:

• Source and a passing test generated from the worked examples. scripts/extension-scaffold-gen.mts reads examples/hello-plugin and examples/iris-theme, rewrites their example-specific names into placeholder tokens through an anchored substitution list (every anchor must match, and no trace of hello/iris may survive — the script fails loudly either way), and emits packages/create-meith/src/extension-templates.ts. pnpm extension:gen:check joins verify and the CI static job, so editing an example is editing the scaffold and the two cannot drift — the board-installer-gen.mts pattern the ticket recommends.
• A workspace that proves itself: package.json depending on the published @meith/plugin-kit (or @meith/theme-kit + @meith/theme-default) at the exact release version, a tsconfig, a vitest setup, and the example's tests exercising a real hook (view.footer) or the resolved theme contract.
• A README walking through local development against a scaffolded board (path install, community.plugins.ts / community.config.ts registration) and marketplace submission.
• A pre-filled listing.json that passes marketplace-gen's validateEntry (asserted in the tests).

The plugin scaffold, unlike the example it comes from, exports the fixed plugin/messages names community plugin:add and board:gen expect, so it is manifest-installable as scaffolded.

Validated

• pnpm verify passes (including the new extension:gen:check gate; 36 new tests).
• Scaffolded both kinds into a temp dir against the published 0.21.2 kits: npm install, npm test, and npm run typecheck all pass for both (the typecheck run is what surfaced the @types/node devDependency the scaffold now carries).
• slots:check learned to skip the generated templates file, which holds a defineTheme( call as string data — documented in docs/contributing/development.md.

🤖 Generated with Claude Code

https://claude.ai/code/session_01G22SRRgkUV7doefmw9NMNf

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