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>

tests/conftest.py

@@ -0,0 +1,20 @@
+"""Command-line options for the repository-level test suites.
+
+Declared here rather than beside the tests that use them: pytest only reads
+options from conftest files it loads at startup, and this directory is one
+of the configured test paths.
+"""
+
+import pytest
+
+
+def pytest_addoption(parser: pytest.Parser) -> None:
+    parser.addoption(
+        "--capability-models",
+        default="",
+        help=(
+            "comma-separated LiteLLM aliases to measure with the capability "
+            "matrix (tests/integration/test_model_capability.py). Spends real "
+            "tokens on every model listed; the matrix skips when this is empty."
+        ),
+    )