Document safe local deployment path for UI mode

[wauputr4]

Goal

Make the UI mode release honest about what is safe today and what must wait for production hardening.

Scope

• Document recommended local-only run mode.
• Document how to bind host/port safely and why public exposure is not recommended yet.
• Document current sandbox meaning: CLI/UI simulation flow, not OS/container isolation.
• Link future hardening work for container/user isolation, network allowlist, secret scoping, and approval enforcement.

Acceptance criteria

• Docs explicitly distinguish UI sandbox/demo from production sandbox isolation.
• README or docs warn against exposing UI mode publicly without auth/reverse proxy/hardening.
• Production-hardening follow-up issues are linked.