navigateTree() treated every custom_message entry as a re-editable user
turn, setting the leaf to the injection's parent and dumping the expanded
skill body into the editor. A /skill:<name> invocation is persisted as a
skill-prompt custom_message, so selecting it in /tree dropped the skill off
the active branch. Skip the parent-leaf/editor-prefill path for
skill-prompt entries so the leaf lands on the injection node itself.
Fixes#5374
roboomp