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>

docs/Tuning-Guide.md

@@ -248,8 +248,8 @@ change one thing at a time.
 1. **Model and quantization.** Start with whether a candidate holds the
    tool-call contract at all:
 
-       NISSE_CAPABILITY_MODELS=alias-a,alias-b \
-         uv run pytest -m integration tests/integration/test_model_capability.py
+       uv run pytest -m integration tests/integration/test_model_capability.py \
+         --capability-models=alias-a,alias-b
 
    A model that falls back to prose instead of calling tools is disqualified,
    however good its benchmarks look. Before comparing aliases, check what