feat(rules): always_on rulebook flag + Scribe-first prompt

Adds rulebooks.always_on (migration 0058) and a new list_always_on_rules
MCP tool so a session-start eager pull can fetch standing rules without
needing an active-project notion. Updates _INSTRUCTIONS so Claude calls
the new tool at session start and codifies engineering rules in Scribe
rather than CLAUDE.md / auto-memory.

Seeds FabledSword family rulebook to always_on=true on migrate, matching
its design role as the cross-project standards rulebook.

Frontend: badge in RulebookListPane for always-on rulebooks; toggle in
RulebookDetailPane header bound to a new toggleAlwaysOn store action.

This is S1+S2 of the rules-consolidation plan (Scribe task #508). S3
(project-scoped rules) and S4 (enter_project handshake) follow.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/fabledassistant/routes/rulebooks.py

@@ -55,7 +55,7 @@ async def get_rulebook(rulebook_id: int):
 @login_required
 async def update_rulebook(rulebook_id: int):
     data = await request.get_json() or {}
-    fields = {k: v for k, v in data.items() if k in ("title", "description")}
+    fields = {k: v for k, v in data.items() if k in ("title", "description", "always_on")}
     rb = await rulebooks_svc.update_rulebook(rulebook_id, _uid(), **fields)
     if rb is None:
         return jsonify({"error": "rulebook not found"}), 404