0006 Detach key, image key and memory cap defaults

Context

simstim intercepts two keys while attached, and caps each session’s memory.

Detach: Ctrl+. Press it twice quickly to send it to the agent. No agent uses it by default.
Image paste: Ctrl+V when the laptop’s clipboard holds an image; otherwise the key passes through. Alt+V on Windows, where the terminal takes Ctrl+V.
Memory cap per session: 8 GiB or 40 % of the host’s memory, whichever is smaller. MemoryHigh is 85 % of that, and swap is off for the session.

All three can be changed in the config.

Good, because a leaking agent cannot starve the host or other sessions.
Bad, because a heavy build inside one session can hit the cap. The error says so, and the cap can be raised.