Add zero-setup release demos and starter templates

[caoergou]

Summary

• add a real 15-second boot-to-story demo GIF and surface CI/bundle badges
• add standalone npm-consumer templates for quick start and custom files + scenario, linked for StackBlitz and CodeSandbox
• add full-session and prologue demo query params with E2E coverage
• complete npm discovery keywords and shorten the README quick start

Scope note

Per maintainer direction, this PR does not publish or draft third-party community posts. External posting is an operational decision, not a repository-verifiable engineering acceptance criterion.

Validation

• npm run examples:check
• npm exec playwright test e2e/branding.spec.ts
• npm run format:check
• npm run lint
• npm run typecheck
• npm run guard:nocheck
• npm run guard:purity
• npm run build:lib
• npm run size:check

The full local Vitest run was affected by existing sandbox/resource limitations (unrelated 5-second timeouts and spawnSync EPERM); GitHub Actions will run it in isolation.

Closes #216