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/integration/conftest.py

@@ -21,8 +21,8 @@ from nisse_dev.llm import AnthropicPassthroughClient, LiteLLMClient
 REPO = Path(__file__).resolve().parents[2]
 TEST_CONFIG = Path(__file__).parent / "nisse-test.yaml"
 
-# At import, so per-run switches such as NISSE_CAPABILITY_MODELS can be set
-# in .env too. Real environment variables win over the file.
+# The secrets nisse-test.yaml refers to, for local runs. CI sets them as real
+# environment variables, which win over the file.
 load_env_file(REPO / ".env")