feat: add update command, bump to 0.2.0, doc version check

- add secrets update: incremental merge for tags/metadata/encrypted
- AGENTS.md: 提交前检查增加版本号与 git tag 说明
- README/AGENTS: update 命令文档与示例
- Cargo.toml 0.1.0 -> 0.2.0 (secrets-0.1.0 已存在)

Made-with: Cursor

src/commands/add.rs

@@ -4,7 +4,7 @@ use sqlx::PgPool;
 use std::fs;
 
 /// Parse "key=value" entries. Value starting with '@' reads from file.
-fn parse_kv(entry: &str) -> Result<(String, String)> {
+pub(crate) fn parse_kv(entry: &str) -> Result<(String, String)> {
     let (key, raw_val) = entry.split_once('=').ok_or_else(|| {
         anyhow::anyhow!(
             "Invalid format '{}'. Expected: key=value or key=@file",