fix: add test_headless_execute.py to _FIXTURE_MEDIATED_ENTRIES in cascade guard

The new test file uses minimal_ctx fixture which imports autoskillit.pipeline
at call time but not at AST-import level — requires exemption in the arch guard's
_FIXTURE_MEDIATED_ENTRIES dict.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: Trecek

tests/arch/test_cascade_map_guard.py

@@ -680,6 +680,7 @@ def test_every_file_level_entry_references_existing_test(self) -> None:
             "execution/test_headless_dispatch.py",
             "execution/test_headless_env_injection.py",
             "execution/test_headless_env_scrub.py",
+            "execution/test_headless_execute.py",
             "execution/test_headless_provider_fallback.py",
             "execution/test_headless_synthesis.py",
             "execution/test_idle_output_env.py",