refactor: 消除冗余、统一设计，bump 0.9.1

- 提取 EntryRow/SecretFieldRow 到 models.rs
- 提取 current_actor()、print_json() 公共函数
- ExportFormat::from_extension 复用 from_str
- fetch_entries 默认 limit 100k（export/inject/run 不再截断）
- history 独立为 history.rs 模块
- delete 改用 DeleteArgs 结构体
- config_dir 改为 Result，Argon2id 参数提取常量
- Cargo 依赖 ^ 前缀、tokio 精简 features
- 更新 AGENTS.md 项目结构

Made-with: Cursor

src/audit.rs

@@ -1,6 +1,11 @@
 use serde_json::Value;
 use sqlx::{Postgres, Transaction};
 
+/// Return the current OS user as the audit actor (falls back to empty string).
+pub fn current_actor() -> String {
+    std::env::var("USER").unwrap_or_default()
+}
+
 /// Write an audit entry within an existing transaction.
 pub async fn log_tx(
     tx: &mut Transaction<'_, Postgres>,
@@ -10,7 +15,7 @@ pub async fn log_tx(
     name: &str,
     detail: Value,
 ) {
-    let actor = std::env::var("USER").unwrap_or_default();
+    let actor = current_actor();
     let result: Result<_, sqlx::Error> = sqlx::query(
         "INSERT INTO audit_log (action, namespace, kind, name, detail, actor) \
          VALUES ($1, $2, $3, $4, $5, $6)",