Adds a persisted, runtime-editable system prompt. The brain reads the persona
on every turn (prompt_store.get_persona), so a dashboard edit applies to the
next reply with no restart; blank clears the override back to the built-in
PERSONA. New endpoints GET/POST /api/prompt, and a reusable modal popup
(뒤로가기 + 수정/저장) that later white/blacklist features will share.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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