feat(display): verbose skill change notifications with content previews

When display.memory_notifications is set to 'verbose', skill_manage
notifications now show meaningful change details instead of just the
generic tool message.

Before (verbose mode):
  💾 📝 Patched SKILL.md in skill 'gogcli' (1 replacement).

After (verbose mode):
  💾 📝 Skill 'gogcli' patched: "old pitfall text..." → "new pitfall text..."

Changes:
- skill_manager_tool.py: _patch_skill() now includes old/new string
  previews (truncated to 200 chars) in the result via '_change' key.
  _create_skill() and _edit_skill() include skill description from
  frontmatter for verbose create/edit notifications.
- run_agent.py: Background review notification builder now reads the
  '_change' dict from skill tool results and formats descriptive
  notifications per action type (patch → old→new diff, create/edit →
  description preview). Falls back to generic message when _change
  data is unavailable (backwards compatible).

This is especially useful when subagents patch skills, since neither
the user nor the parent agent can see what the subagent changed.

run_agent.py

@@ -1411,10 +1411,15 @@ class AIAgent:
     def _summarize_background_review_actions(
         review_messages: List[Dict],
         prior_snapshot: List[Dict],
+        notification_mode: str = "on",
     ) -> List[str]:
         """Forwarder — see ``agent.background_review.summarize_background_review_actions``."""
         from agent.background_review import summarize_background_review_actions
-        return summarize_background_review_actions(review_messages, prior_snapshot)
+        return summarize_background_review_actions(
+            review_messages,
+            prior_snapshot,
+            notification_mode=notification_mode,
+        )
 
     def _spawn_background_review(
         self,