feat: add interactive env setup wizard

[embire2]

Summary

This adds an optional, dependency-free setup wizard to make initial local configuration more "autonomous" and repeatable (env init + prompts), per #2101.

What’s Included

• pnpm run setup (interactive by default)
• scripts/setup.mjs:
  • --init create .env.local from .env.example (no overwrite unless --force)
  • --sync copy .env.local → .env (Docker Compose compatibility)
  • --list list known env keys parsed from .env.example
  • --set KEY=VALUE / --unset KEY for non-interactive updates
  • interactive prompts for any keys (with hidden input for likely-secret keys)
• README: a short pointer to the setup wizard in the Docker env section

Why

New installs currently require manual copying/editing of env files. This change adds a single, discoverable command that can:

• initialize the expected env files
• optionally sync for Docker
• prompt for keys/values in one place

This is intentionally lightweight and avoids new dependencies.

Testing

• pnpm test
• Local end-to-end smoke: start dev server and POST /api/chat using OpenAI with a chat-capable model (gpt-4o), verifying streamed output contained the expected marker.

Closes #2101.