A capability image is ~7 GB and nearly all of it is torch, pulled again by
every cold pod. With a network volume those libraries can live there
instead: `venv_on_volume` boots a slim image — OS, Python and coderai
only — and runs coderai from a venv on the volume. The first pod builds
it; every pod after skips both the pull and the install.

Opt-in per pool and per model, never a default, because the trade is real
and unmeasured: the pull gets much smaller, but importing torch off
network storage is slower than off local disk, so some of the saving
comes back at import time. Whether that is a net win depends on how often
pods cold-start, which is the user's workload, not mine to assume.

Two hazards are handled rather than left to chance. The ready marker is
written AFTER the install, so a pod that dies mid-build leaves nothing a
later pod would import from. And a lock directory — mkdir is atomic —
makes a second pod wait for the first instead of two installs
interleaving into the same directory.

`venv_on_volume` without a volume is refused with the reason, rather than
booting a pod that installs 7 GB onto container disk and discards it.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0182DhR1eFNPQyGmDrpHbedY