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/Cargo.lock

@@ -491,6 +491,7 @@ dependencies = [
  "codex-common",
  "codex-core",
  "codex-exec",
+ "codex-mcp-server",
  "codex-tui",
  "serde_json",
  "tokio",