docs: update README for Phase 3 infrastructure completion

Update README to reflect completed Phase 3 infrastructure layer:
- Documented ModbusRelayController, MockRelayController, SqliteRelayLabelRepository, and HealthMonitor implementations
- Added testing coverage details (20+ tests across infrastructure components)
- Updated architecture diagrams and project structure
- Changed task reference to tasks.org format
- Updated dependency list with production infrastructure dependencies

Ref: Phase 3 tasks in specs/001-modbus-relay-control/tasks.org

README.md

@@ -1,4 +1,18 @@
-# STA - Smart Temperature & Appliance Control
+<h1 align="center">STA</h1>
+<div align="center">
+ <strong>
+   Smart Temperature & Appliance Control
+ </strong>
+</div>
+<br/>
+
+<div align="center">
+  <!-- Wakapi -->
+  <img alt="Coding Time Badge" src="https://clock.phundrak.com/api/badge/phundrak/interval:any/project:sta">
+  <!-- Emacs -->
+  <a href="https://www.gnu.org/software/emacs/"><img src="https://img.shields.io/badge/Emacs-30.2-blueviolet.svg?style=flat-square&logo=GNU%20Emacs&logoColor=white" /></a>
+</div>
+<br/>
 
 > **🤖 AI-Assisted Development Notice**: This project uses Claude Code as a development assistant for task planning, code organization, and workflow management. However, all code is human-written, reviewed, and validated by the project maintainer. AI is used as a productivity tool, not as the author of the implementation.