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/mcp-server/Cargo.toml

@@ -3,6 +3,14 @@ name = "codex-mcp-server"
 version = { workspace = true }
 edition = "2024"
 
+[[bin]]
+name = "codex-mcp-server"
+path = "src/main.rs"
+
+[lib]
+name = "codex_mcp_server"
+path = "src/lib.rs"
+
 [lints]
 workspace = true