Pass the capability matrix's models as a pytest option

NISSE_CAPABILITY_MODELS was neither a secret nor configuration: which
models to measure is a question for one run, so it is now an argument,
--capability-models=a,b, declared in tests/conftest.py where pytest
reads options at startup. With no models the matrix skips as before, so
CI is unchanged. The last non-secret NISSE_* variable other than
NISSE_CONFIG and NISSE_ENV is gone.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

.env.example

@@ -12,7 +12,3 @@ NISSE_GITEA_TOKEN=your-scoped-service-account-token
 # Where the config file is, if not ./nisse-dev.yaml. Where this file is, if
 # not ./.env, is NISSE_ENV, which has to be set in the real environment.
 # NISSE_CONFIG=~/nisse-demo-workspace/nisse-dev.yaml
-
-# Opt-in for the capability matrix: models to measure, comma separated.
-# Spends real tokens across every model listed. See docs/Tuning-Guide.md.
-# NISSE_CAPABILITY_MODELS=local-fast,local-coder