fix(cli): correct mcp add usage order (#6827)

## Summary
- add an explicit `override_usage` string to `AddArgs` so clap prints
`<NAME>` before the command/url choice, matching the actual parser and
docs

### Before

Usage: codex mcp add [OPTIONS] <COMMAND|--url <URL>> <NAME>


### After

Usage: codex mcp add [OPTIONS] <NAME> [--url <URL> | -- <COMMAND>...]

---------

Signed-off-by: kyuheon-kr <kyuheon.kr@gmail.com>

codex-rs/cli/src/mcp_cmd.rs

@@ -79,6 +79,7 @@ pub struct GetArgs {
 }
 
 #[derive(Debug, clap::Parser)]
+#[command(override_usage = "codex mcp add [OPTIONS] <NAME> (--url <URL> | -- <COMMAND>...)")]
 pub struct AddArgs {
     /// Name for the MCP server configuration.
     pub name: String,