feat: Optimize TODO_LIST structure with hierarchical display and container task handling

- Replace separate Main Tasks/Subtasks sections with unified hierarchical list
- Use ▸ symbol for container tasks (with subtasks) instead of checkboxes
- Maintain standard - [ ]/- [x] for executable leaf tasks
- Add 2-space indentation to show task hierarchy clearly
- Include status legend for better user understanding

Benefits:
- Eliminates confusion about non-executable main tasks
- Provides clear visual hierarchy in single list
- Reduces TODO_LIST complexity and improves usability
- Aligns container task concept with execution model

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

.claude/agents/code-developer.md

@@ -122,14 +122,18 @@ ELIF context insufficient OR task has analysis marker:
    ```markdown
    # Tasks: User Authentication System
    
-   ## Main Tasks
-   - [x] **IMPL-001**: Create auth module → [📋](./.task/impl-001.json) | [✅](./.summaries/IMPL-001.md)
+   ## Task Progress
+   ▸ **IMPL-001**: Create auth module → [📋](./.task/impl-001.json)
+     - [x] **IMPL-001.1**: Database schema → [📋](./.task/impl-001.1.json) | [✅](./.summaries/IMPL-001.1.md)
+     - [ ] **IMPL-001.2**: API endpoints → [📋](./.task/impl-001.2.json)
+   
    - [ ] **IMPL-002**: Add JWT validation → [📋](./.task/impl-002.json)
    - [ ] **IMPL-003**: OAuth2 integration → [📋](./.task/impl-003.json)
    
-   ## Subtasks
-   - [x] **IMPL-001.1**: Database schema → [📋](./.task/impl-001.1.json) | [✅](./.summaries/IMPL-001.1.md)
-   - [ ] **IMPL-001.2**: API endpoints → [📋](./.task/impl-001.2.json)
+   ## Status Legend
+   - `▸` = Container task (has subtasks)
+   - `- [ ]` = Pending leaf task
+   - `- [x]` = Completed leaf task
    ```
 
 3. **Generate Summary** (using session context paths):