fix(health): suppress 404 when health monitor disabled + fix audit test assertion

- Sidebar.svelte: defer healthStore.refresh() until feature flags confirm
  health_monitor is enabled; skip entirely when disabled
- health.js: remove throw error from catch block — log and return null
  instead, preventing 'Uncaught (in promise)' on expected 404
- test_report_audit_immutability.py: fix mock assertions —
  audit_service uses logger.reason/reflect/explore, not logger.info
- HealthStore and Sidebar now produce zero network noise when
  FEATURES__HEALTH_MONITOR=false

backend/git_repos/remote/test-repo.git/hooks/pre-merge-commit.sample

@@ -0,0 +1,13 @@
+#!/bin/sh
+#
+# An example hook script to verify what is about to be committed.
+# Called by "git merge" with no arguments.  The hook should
+# exit with non-zero status after issuing an appropriate message to
+# stderr if it wants to stop the merge commit.
+#
+# To enable this hook, rename this file to "pre-merge-commit".
+
+. git-sh-setup
+test -x "$GIT_DIR/hooks/pre-commit" &&
+        exec "$GIT_DIR/hooks/pre-commit"
+: