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.

tests/run_agent/test_background_review.py

@@ -115,10 +115,11 @@ def test_background_review_summarizer_receives_captured_messages_after_close(mon
             # must have snapshot them before this runs.
             self._session_messages = []
 
-    def fake_summarize(review_messages, prior_snapshot):
+    def fake_summarize(review_messages, prior_snapshot, notification_mode="on"):
         events.append("summarize")
         captured["review_messages"] = list(review_messages)
         captured["prior_snapshot"] = list(prior_snapshot)
+        captured["notification_mode"] = notification_mode
         return []
 
     monkeypatch.setattr(run_agent_module, "AIAgent", FakeReviewAgent)
@@ -146,6 +147,7 @@ def test_background_review_summarizer_receives_captured_messages_after_close(mon
     ]
     assert captured["review_messages"] == [review_tool_message]
     assert captured["prior_snapshot"] == messages_snapshot
+    assert captured["notification_mode"] == "on"
 
 
 def test_background_review_installs_auto_deny_approval_callback(monkeypatch):