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/Cargo.toml

@@ -24,6 +24,7 @@ clap = { version = "4", features = ["derive"] }
 codex-core = { path = "../core" }
 codex-common = { path = "../common", features = ["cli"] }
 codex-exec = { path = "../exec" }
+codex-mcp-server = { path = "../mcp-server" }
 codex-tui = { path = "../tui" }
 serde_json = "1"
 tokio = { version = "1", features = [