feat(776): every hook reports that it fired, and the report is measured (#795)

Co-authored-by: Timothy <timothy@noreply.gitea.tblindustries.be>

scripts/tests/conftest.py

@@ -0,0 +1,29 @@
+"""Keep the test suite out of the PRODUCTION hook-fire log (ersatztv#776).
+
+Four suites here drive real hooks as subprocesses — `test_merge_consent_exemption.py`,
+`test_merge_consent_base_change.py`, `test_prepush_rebase_check_tag_exemption.py`,
+`test_pr_changed_files.py` — and they predate the instrumentation. Once every hook records its own
+execution, running `pytest scripts/tests` wrote its synthetic invocations into
+`$HOME/.cache/ersatztv/hook-fire/`: 96 `pretooluse-merge-consent` fires including two `deny`s, and
+12 `blocked` decisions from `prepush-rebase-check`, none of them a real session.
+
+That is not untidiness, it is the defect the whole change exists to remove. `scripts/hook-fire-log.sh
+report` is meant to answer "what did the harness actually do", and a log carrying test artifacts
+answers a different question while looking identical — inference reintroduced one layer up. The
+record has no field distinguishing a test invocation from a real one, and adding one would only move
+the problem, so the fix is that tests never write to the real log at all.
+
+Autouse and session-independent: a new suite that drives a hook is isolated by existing, without
+having to know this file is here.
+"""
+
+from __future__ import annotations
+
+import pytest
+
+
+@pytest.fixture(autouse=True)
+def isolate_hook_fire_log(tmp_path_factory, monkeypatch):
+    monkeypatch.setenv(
+        "ETV_HOOK_FIRE_LOG_DIR", str(tmp_path_factory.mktemp("hook-fire-log"))
+    )