refactor(mqlaunch): extract prompt concern

[MCamner]

Why

Step 11a reduces the live mqlaunch monolith one concern at a time. Prompt
directory handling, AI-backend guardrails, prompt browsing, and prompt backups
were still defined directly in the launcher, which made their ownership harder
to test and allowed duplicate definitions to return unnoticed.

What Changed

• Move the seven prompt and AI-backend functions into the shared zsh library
  mqlaunch/lib/prompts.sh.
• Source that library from the live launcher without changing command or menu
  routes.
• Add a focused contract test for prompt-directory precedence, backend status,
  and AI-mode argument forwarding.
• Register the concern in the monolith de-layering gate and full selftest.
• Document the scoped implementation and rollback plan.

Verification

The focused contract test confirms that the repo-local prompt directory still
wins over the configured fallback, all three backend states are preserved, and
safe_run_ai forwards the selected mode unchanged. The monolith gate confirms
that all seven functions have one live definition.