fix potential negative calculation crash

[NotoriousPyro]

time.monotonic() is called once for the while condition and again for sleep(). If the deadline passes between those calls, until - time.monotonic() becomes negative.

I changed the loop to calculate one remaining duration per iteration and break cleanly when it reaches zero.

This prevents the following rare crash:

{
  "type": "ValueError",
  "reason": "sleep length must be non-negative",
  "locations": [
    "cli.py:333 main"
  ]
}