🤖 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>

scripts/build.sh

@@ -18,9 +18,15 @@ go build -o bin/server ./cmd/server
 echo "📦 Building greet CLI..."
 go build -o bin/greet ./cmd/greet
 
+# Build new Cobra CLI binary
+echo "📦 Building Cobra CLI..."
+go build -o bin/dance-lessons-coach ./cmd/cli
+
 echo "✅ Build complete!"
-echo "   Server binary: ./bin/server"
-echo "   Greet binary:  ./bin/greet"
+echo "   Server binary:       ./bin/server"
+echo "   Greet binary:        ./bin/greet"
+echo "   Cobra CLI binary:    ./bin/dance-lessons-coach"
 echo ""
 echo "💡 To run the server: ./bin/server"
 echo "💡 To use the greet CLI: ./bin/greet [name]"
+echo "💡 To use the Cobra CLI: ./bin/dance-lessons-coach --help"