Add a command to enable or disable headroom live, globally or for one session #400

agent-shim headroom enable|disable changes the live global setting the door reads per request, applying to new sessions by default, and --session <id> overrides one session. A change to a session already using headroom needs an explicit --now and prints that it will rewrite that session's prompt cache once. The precedence is the per-session override, then the live global setting, then the launch default.