Add mock file audit backend

Makefile

@@ -0,0 +1,24 @@
+SHELL := /usr/bin/env bash
+.DEFAULT_GOAL := help
+
+AUDIT_CORE_MOCK_DIR ?= /tmp/audit-core
+
+test: ## Run unit tests
+	python3 -m pytest -q
+
+mock-audit-smoke: ## Write one non-secret smoke event to the mock audit backend
+	AUDIT_CORE_MOCK_DIR="$(AUDIT_CORE_MOCK_DIR)" python3 -m audit_core emit \
+	  --source audit-core \
+	  --action audit_core.mock_backend.smoke \
+	  --resource "$(AUDIT_CORE_MOCK_DIR)" \
+	  --outcome success \
+	  --detail backend=mock-file
+
+mock-audit-cleanup: ## Remove mock audit files older than the retention window
+	AUDIT_CORE_MOCK_DIR="$(AUDIT_CORE_MOCK_DIR)" python3 -m audit_core cleanup
+
+help: ## Show this help
+	@awk 'BEGIN {FS = ":.*##"; printf "\nUsage:\n  make \033[36m<target>\033[0m\n"} \
+	  /^[a-zA-Z_-]+:.*?##/ { printf "  \033[36m%-24s\033[0m %s\n", $$1, $$2 }' $(MAKEFILE_LIST)
+
+.PHONY: test mock-audit-smoke mock-audit-cleanup help