Flag planned work on one monitor agent's server: no incident, no alert, no downtime

Accepts an account API key with read-write access to monitors, or the agent's own token with id set to the agent's id or to self. While the window is active the agent's offline and server-health alerts are withheld, and every check that runs from the agent, or whose target host is in hosts or is the host name the agent reports, opens no incident, sends nothing, and does not count against uptime. When the window ends, evaluation resumes from that moment: anything still down is reported then, never backdated. Posting while a window is active extends it to now plus minutes and adds the new hosts.