feat(dashboard): live-editable bot prompt via popup (view/edit/save)

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>

wsai/prompt_store.py

@@ -0,0 +1,54 @@
+"""Persisted, live-editable bot persona (the brain's system prompt).
+
+The dashboard lets the user view and edit the bot's system prompt at runtime.
+The brain reads the current persona on every turn, so an edit takes effect on
+the next reply with no restart. The text is persisted to disk so it survives a
+restart; when no override file exists the caller's default
+(``ClaudeBrain.PERSONA``) is used.
+
+Path: ``$WSAI_PROMPT_PATH`` or ``~/.config/wsai/persona.txt`` (disk, never a
+RAM-backed tmpfs).
+"""
+
+from __future__ import annotations
+
+import os
+import threading
+from pathlib import Path
+
+_LOCK = threading.Lock()
+
+
+def _path() -> Path:
+    override = os.environ.get("WSAI_PROMPT_PATH")
+    return Path(override) if override else (Path.home() / ".config" / "wsai" / "persona.txt")
+
+
+def get_persona(default: str) -> str:
+    """Return the saved persona override, or ``default`` if none is set."""
+    try:
+        with _LOCK:
+            text = _path().read_text(encoding="utf-8")
+    except OSError:
+        return default
+    return text if text.strip() else default
+
+
+def set_persona(text: str) -> None:
+    """Persist a new persona. Blank text clears the override (reverts to default)."""
+    p = _path()
+    with _LOCK:
+        p.parent.mkdir(parents=True, exist_ok=True)
+        if text and text.strip():
+            p.write_text(text, encoding="utf-8")
+        else:
+            p.unlink(missing_ok=True)
+
+
+def is_overridden() -> bool:
+    """True when a non-empty override file is in effect."""
+    try:
+        with _LOCK:
+            return bool(_path().read_text(encoding="utf-8").strip())
+    except OSError:
+        return False