chore: add per-environment env config (参考 uirefbase)

Commit .env.development/.staging/.production and track env files in git
(matching uirefbase convention). No secrets: SSO_URL (sso-mock service),
NEXT_PUBLIC_APP_URL (callback), SSO_APP_ID. .gitignore now ignores only
*.local overrides.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.gitignore

@@ -28,10 +28,8 @@ yarn-debug.log*
 yarn-error.log*
 .pnpm-debug.log*
 
-# env files (never commit env; provide .env.example as a template instead)
-.env
-.env.*
-!.env.example
+# env files — committed per project convention (参考 uirefbase)；仅忽略本地覆盖文件
+.env*.local
 
 # vercel
 .vercel