feat: add mcp subcommand to CLI to run Codex as an MCP server (#934)

Previously, running Codex as an MCP server required a standalone binary
in our Cargo workspace, but this PR makes it available as a subcommand
(`mcp`) of the main CLI.

Ran this with:

```
RUST_LOG=debug npx @modelcontextprotocol/inspector cargo run --bin codex -- mcp
```

and verified it worked as expected in the inspector at
`http://127.0.0.1:6274/`.

codex-rs/cli/src/main.rs

@@ -33,6 +33,9 @@ enum Subcommand {
     #[clap(visible_alias = "e")]
     Exec(ExecCli),
 
+    /// Experimental: run Codex as an MCP server.
+    Mcp,
+
     /// Run the Protocol stream via stdin/stdout
     #[clap(visible_alias = "p")]
     Proto(ProtoCli),
@@ -70,6 +73,9 @@ async fn main() -> anyhow::Result<()> {
         Some(Subcommand::Exec(exec_cli)) => {
             codex_exec::run_main(exec_cli).await?;
         }
+        Some(Subcommand::Mcp) => {
+            codex_mcp_server::run_main().await?;
+        }
         Some(Subcommand::Proto(proto_cli)) => {
             proto::run_main(proto_cli).await?;
         }