fix: guard set -e footgun in _fast_copy_dir and reset cache in tests

- Use if/fi instead of && for partial-clone cleanup to avoid exit
  code 1 leaking under set -e when target doesn't exist
- Reset _fast_copy_os in test setup for deterministic test isolation

Author: helizaga

lib/copy.sh

@@ -97,7 +97,7 @@ _fast_copy_dir() {
       fi
       # Clean up any partial clone output before fallback
       local _clone_target="${dest%/}/$(basename "$src")"
-      [ -e "$_clone_target" ] && rm -rf "$_clone_target"
+      if [ -e "$_clone_target" ]; then rm -rf "$_clone_target"; fi
       cp -RP "$src" "$dest"
       ;;
     linux)

tests/copy_safety.bats

@@ -2,6 +2,7 @@
 
 setup() {
   load test_helper
+  _fast_copy_os=""
   source "$PROJECT_ROOT/lib/platform.sh"
   source "$PROJECT_ROOT/lib/copy.sh"
 }