fix(readme): Update status to complete and add completion details for Mermaid Diagram Showcase

Author: tikazyq

specs/120-mermaid-diagram-showcase/README.md

@@ -1,5 +1,5 @@
 ---
-status: planned
+status: complete
 created: '2025-11-25'
 tags:
   - test
@@ -10,12 +10,17 @@ priority: medium
 created_at: '2025-11-25T12:52:24.676Z'
 related:
   - 119-ui-diagram-rendering
-updated_at: '2025-11-25T12:53:39.169Z'
+updated_at: '2025-11-26T08:24:39.899Z'
+completed_at: '2025-11-26T08:24:39.899Z'
+completed: '2025-11-26'
+transitions:
+  - status: complete
+    at: '2025-11-26T08:24:39.899Z'
 ---
 
 # Mermaid Diagram Showcase
 
-> **Status**: 🗓️ Planned · **Priority**: Medium · **Created**: 2025-11-25 · **Tags**: test, ui, mermaid, visualization
+> **Status**: ✅ Complete · **Priority**: Medium · **Created**: 2025-11-25 · **Tags**: test, ui, mermaid, visualization
 
 **Project**: lean-spec  
 **Team**: Core Development