chore: Remove unused secret/ scaffolding

The project has no runtime or compile-time secrets (offline desktop
tool, no DB/API keys), so the secret/ folder held only example launch
scripts that were never used. Remove it and the related .gitignore
rules; trim references in ARCHITECTURE.md and CLAUDE.md.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitignore

@@ -5,12 +5,6 @@ publish/
 *.user
 .vs/
 
-# secret/ 全部忽略，但保留 README 與範本
-secret/*
-!secret/README.md
-!secret/*.example
-!secret/.gitkeep
-
 # AI 協作素材不入 git
 For_AI/