[tdene]

skip_venv_if_present trusts based on the existence of bin/python and bin/activate.

Both these files appear long before the venv is actually built. This allows for zombie venvs to be left behind which pass the skip check but are unusable.

This PR adds a completion marker.

[tdene]

/ok to test e04eacf

[ananthsub]

/claude review

[claude]

SHIP — no reliability concerns.

Correctly closes the mid-install-death gap in setup_env_command. uv venv creates bin/python and bin/activate in seconds, so the old skip check (skip_venv_if_present + those two files) would reuse a venv whose uv pip install had crashed halfway — a silently-incomplete environment that later fails obscurely at import/runtime.

The fix is sound:

• .nemo_gym_venv_complete is touched only as the final &&-gated step, so it appears only on a fully-installed venv.
• rm -f clears it first in the rebuild chain, so a death anywhere before touch leaves no marker and the next run rebuilds instead of skipping.
• Skip now additionally requires the marker.

Backward compat is fine: pre-existing markerless venvs rebuild once (uv venv --allow-existing absorbs the re-create), a one-time cost rather than any correctness risk. Tests updated across all command-shape assertions plus a dedicated regression test for the partial-build case. No verifier/scorer, async, config, or dependency surface touched.

[tdene]

/ok to test 45b436a

[tdene]

@ananthsub I've rebased, and the solution is now much simpler. Please take a look; this PR is very thin now.