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/tui/src/slash_command.rs

@@ -7,7 +7,9 @@ use strum_macros::EnumString;
 use strum_macros::IntoStaticStr;
 
 /// Commands that can be invoked by starting a message with a leading slash.
-#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, EnumString, EnumIter, AsRefStr, IntoStaticStr)]
+#[derive(
+    Debug, Clone, Copy, PartialEq, Eq, Hash, EnumString, EnumIter, AsRefStr, IntoStaticStr,
+)]
 #[strum(serialize_all = "kebab-case")]
 pub enum SlashCommand {
     Clear,