🤖 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>

README.md

@@ -24,7 +24,13 @@ A Go project demonstrating idiomatic package structure, CLI implementation, and
 git clone https://github.com/yourusername/DanceLessonsCoach.git
 cd DanceLessonsCoach
 
-# Build and run
+# Build all binaries
+./scripts/build.sh
+
+# Use the new Cobra CLI
+./bin/dance-lessons-coach --help
+
+# Or use the legacy greet CLI
 go run ./cmd/greet
 ```
 
@@ -54,7 +60,23 @@ export DLC_LOGGING_JSON=true
 
 ## Usage
 
-### CLI
+### New Cobra CLI (Recommended)
+
+```bash
+# Show help
+./bin/dance-lessons-coach --help
+
+# Show version
+./bin/dance-lessons-coach version
+
+# Greet someone
+./bin/dance-lessons-coach greet John
+
+# Start server
+./bin/dance-lessons-coach server
+```
+
+### Legacy CLI (Deprecated)
 
 ```bash
 # Default greeting