feat(core,exporters,desktop): end-to-end first demo + HTML export

[hqhq1025]

Goal

Make the README's "first demo" actually work end-to-end IF the user provides
an API key:

1. Pick the Calm Spaces meditation app starter
2. Send → an HTML mockup renders in the sandboxed iframe
3. Click Export → HTML → save → opens in the browser as a self-contained file
4. Click Export → PDF / PPTX / ZIP → loud toast: "… ships in Phase 2"

Files in

New

• packages/templates/src/system/index.ts — SYSTEM_PROMPTS.designGenerator
• packages/templates/src/system/design-generator.md — reviewable prompt source
• packages/exporters/src/html.ts — real exportHtml() with Tailwind injection + prettifier
• packages/exporters/src/{pdf,pptx,zip}.ts — Phase-2 stubs that throw CodesignError(EXPORTER_NOT_READY)
• apps/desktop/src/main/exporter-ipc.ts — codesign:export handler over dialog.showSaveDialog
• apps/desktop/src/renderer/src/components/PreviewToolbar.tsx — Export dropdown + toast slot
• packages/core/src/generate.test.ts — vitest with mocked provider
• examples/calm-spaces/README.md — demo doc

Modified

• packages/core/src/index.ts — pulls system prompt from templates; collapsed duplicated artifact-extraction loop into a collect() helper
• packages/exporters/src/index.ts + package.json — top-level exportArtifact() that lazy-imports format modules; ./html subpath export added
• apps/desktop/src/main/index.ts — one new line: registerExporterIpc(() => mainWindow)
• apps/desktop/src/preload/index.ts — single new method: export(payload)
• apps/desktop/src/renderer/src/store.ts — additive: VITE_OPEN_CODESIGN_DEV_KEY fallback + exportActive(format) + toast slot
• apps/desktop/src/renderer/src/App.tsx — single import + <PreviewToolbar /> above the iframe (layout untouched for wt/preview-ux)

Files NOT touched

Per worktree boundary: apps/desktop/src/main/{config,keychain,onboarding-ipc,locale-ipc}.ts,
apps/desktop/src/renderer/src/onboarding/**, all preview-ux components
(Toast, Settings, CommandPalette, ThemeToggle, TopBar, Sidebar,
PreviewPane, preview/{Empty,Loading,Error}State.tsx, hooks/useKeyboard.ts),
packages/i18n/**, packages/ui/src/components/{IconButton,Tooltip}.tsx,
packages/{runtime,providers,artifacts,shared}/**.

NO new prod deps

Confirmed:

• packages/exporters/package.json — no new dep
• packages/templates/package.json — no new dep
• packages/core/package.json — only added @open-codesign/templates (workspace internal)
• apps/desktop/package.json — only added @open-codesign/exporters (workspace internal); uses Electron's built-in dialog

Lockfile diff is workspace edges only. Install size budget unaffected.

Acceptance test outcomes

• pnpm install clean
• pnpm -r typecheck green across 9 workspaces (incl. desktop main + web)
• pnpm lint green (biome check, 61 files, 0 errors, 0 warnings)
• pnpm -r test green: 3 new tests in packages/core/src/generate.test.ts (empty-prompt error, artifact extraction shape, system-prompt wiring) + existing artifact + runtime suites
• Manual smoke (renderer wiring): VITE_OPEN_CODESIGN_DEV_KEY is read at build time via import.meta.env, falls through to existing assistant-message guidance when unset

A reference exported sample will be attached to this PR thread once a key is plumbed in CI; the IPC + exporter logic is unit-covered today.

Integration notes for parallel worktrees

wt/onboarding — when real keychain plumbing lands:

1. In apps/desktop/src/renderer/src/store.ts, replace the DEV_API_KEY constant + the if (!apiKey) guidance branch with the keychain-fetched key (await the new IPC).
2. Delete the VITE_OPEN_CODESIGN_DEV_KEY mention in examples/calm-spaces/README.md.
3. The existing fallback is intentionally guarded by import.meta.env, so leaving the env var unset is harmless until that integration ships.

wt/preview-ux — PreviewToolbar lives in apps/desktop/src/renderer/src/components/, the directory it claims. If it migrates to a PreviewPane shell, simply move the single <PreviewToolbar /> JSX line into the new container. The component itself depends only on the store; no layout assumptions baked in. The toast slot in the store (toastMessage / dismissToast) deliberately mirrors what the Toast component will need so the two can be merged trivially.

wt/i18n — all user-visible strings in PreviewToolbar and the examples/ README are short and English-only; they're easy to swap to t() once the i18n keys exist. The Phase-2 messaging ("… ships in Phase 2") is a stable string suitable for the i18n catalogue.

Hard rules checklist

• No silent fallbacks — every error path throws CodesignError with a structured code
• All UI uses var(--color-*) tokens; no hardcoded colors / fonts / px
• TypeScript strict; no any; bracket notation; import type for types
• Conventional commit, signed (DCO)
• No new prod deps
• pnpm -r typecheck && pnpm lint && pnpm -r test all green from worktree root