Improve remote-control daemon UX

[owenlin0]

Why

codex remote-control manages the app-server daemon with remote_control enabled, but it previously only exposed an implicit start path. Once started, there was no obvious top-level remote-control command for stopping the daemon; users had to know about the lower-level codex app-server daemon stop command.

The startup failure for missing managed installs was also ambiguous. codex remote-control and daemon bootstrap require the standalone Codex install under CODEX_HOME/packages/standalone/current/codex, but the old error only said to install Codex first, which is unclear when another codex binary is already on PATH. Now we add an explicit instruction for how to get the standalone Codex install.

What changed

• Converts codex remote-control into a command group while preserving bare codex remote-control as the existing start behavior.
• Adds codex remote-control start as the explicit start path.
• Adds codex remote-control stop, which maps to app-server daemon stop.
• Updates the shared daemon managed-install error to name the missing standalone path, explain why that install is required, provide the installer command, and tell users to rerun the command they just tried.

Verification

• cargo test -p codex-app-server-daemon
• cargo test -p codex-cli
• ./target/debug/codex remote-control --help