Proposal: include a pairing code when starting remote control

Problem statement

codex remote-control start enables remote control but does not return a manual pairing code. Users must run codex remote-control pair as a second command, which makes initial setup less discoverable and adds an unnecessary gap between enabling remote control and connecting a controller. The foreground form has the same limitation.

Prototype fix

After remote control becomes ready, the CLI requests a short-lived pairing artifact with a manual code and includes it in the start output. Human output prints the pairing code, while JSON output includes the pairing object. The existing codex remote-control pair command remains available, and the start help text now documents the behavior.

Prototype: https://github.com/Maaarcocr/codex/commit/5fc1f4f9f

Validation: just test -p codex-cli (294 tests passed); just test -p codex-app-server-daemon (30 tests passed).