feat: 开源准备与 upgrade URL 构建时配置

- upgrade: SECRETS_UPGRADE_URL 改为构建时优先（option_env!），CI 自动注入
- upgrade: 支持运行时回退（.env/export），添加 dotenvy 加载 .env
- 泛化示例：IP/实例 ID/域名/密钥名改为示例值（10.0.0.1、example.com 等）
- tasks.json: 文件 secret 测试改用 test-fixtures/example-key.pem
- 文档更新：AGENTS.md、README.md

Made-with: Cursor

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "secrets"
-version = "0.9.1"
+version = "0.9.2"
 edition = "2024"
 
 [dependencies]
@@ -10,6 +10,7 @@ argon2 = { version = "^0.5.3", features = ["std"] }
 chrono = { version = "^0.4.44", features = ["serde"] }
 clap = { version = "^4.6.0", features = ["derive"] }
 dirs = "^6.0.0"
+dotenvy = "^0.15"
 flate2 = "^1.1.9"
 keyring = { version = "^3.6.3", features = ["apple-native", "windows-native", "linux-native"] }
 rand = "^0.10.0"