🤖 feat: implement Cobra CLI with subcommands

- Add new CLI structure in cmd/cli/
- Implement version, server, and greet commands
- Update build script to compile new CLI binary
- Add Cobra dependency to go.mod
- Update ADR 0015 to reflect implementation status
- Update README and AGENTS.md with CLI usage
- Maintain backward compatibility with existing binaries

Generated by Mistral Vibe.
Co-Authored-By: Mistral Vibe <vibe@mistral.ai>

AGENTS.md

@@ -134,14 +134,47 @@ DanceLessonsCoach/
 └── .gitignore          # Ignore patterns
 ```
 
-## 🚀 Server Management
+## 🎮 CLI Management
+
+### New Cobra CLI (Recommended)
+
+DanceLessonsCoach now includes a modern CLI built with Cobra framework:
+
+```bash
+# Show help and available commands
+./bin/dance-lessons-coach --help
+
+# Show version information
+./bin/dance-lessons-coach version
+
+# Greet someone by name
+./bin/dance-lessons-coach greet John
+
+# Start the server
+./bin/dance-lessons-coach server
+```
+
+**Available Commands:**
+- `version` - Print version information
+- `server` - Start the DanceLessonsCoach server
+- `greet [name]` - Greet someone by name
+- `help` - Built-in help system
+- `completion` - Generate shell completion scripts
+
+**Server Command Flags:**
+- `--config` - Config file path
+- `--env` - Environment (dev, staging, prod)
+- `--debug` - Enable debug logging
 
 ### Version Information
 
 The server provides runtime version information:
 
 ```bash
-# Check version
+# Check version using new CLI
+./bin/dance-lessons-coach version
+
+# Check version using server binary
 ./bin/server --version
 
 # Output: