Function prepend_user_hook_array 

fn prepend_user_hook_array(
    hooks_obj: &mut Map<String, Value>,
    key: &str,
    user_value: Value,
)

Merge a user-supplied settings.json-level hook-array (PreToolUse or PreCompact) with whatever is already staged in hooks_obj under key (our auto-injected entries, possibly already carrying legacy content_map["hooks"]-merged user entries from the earlier pass in build_settings_with_hooks). User entries are PREPENDED so their matchers/gates get first refusal; AgentMux’s own entries always stay last. A non-array value is warned and dropped rather than silently discarded — same discipline as every other branch in this function (reagent P1 on PR #813).