🤖 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>

adr/0015-cli-subcommands-cobra.md

@@ -1,10 +1,10 @@
 # 15. CLI Subcommands and Flag Management with Cobra
 
 **Date:** 2026-04-05
-**Status:** 🟡 Proposed
+**Status:** ✅ Implemented
 **Authors:** DanceLessonsCoach Team
-**Decision Date:** TBD
-**Implementation Status:** Not Started
+**Decision Date:** 2026-04-05
+**Implementation Status:** Phase 1 Complete
 
 ## Context
 
@@ -44,7 +44,10 @@ We will adopt **Cobra** as our CLI framework. Cobra is a mature, widely-used lib
 
 ### Implementation Plan
 
-#### Phase 1: Basic Integration (Next Sprint)
+#### Phase 1: Basic Integration (✅ COMPLETED)
+
+**Implemented in:** `cmd/cli/main.go`
+
 ```go
 var rootCmd = &cobra.Command{
     Use:   "dance-lessons-coach",
@@ -80,10 +83,24 @@ var serverCmd = &cobra.Command{
     },
 }
 
+var greetCmd = &cobra.Command{
+    Use:   "greet [name]",
+    Short: "Greet someone by name",
+    Args:  cobra.MaximumNArgs(1),
+    Run: func(cmd *cobra.Command, args []string) {
+        name := ""
+        if len(args) > 0 {
+            name = args[0]
+        }
+        fmt.Printf("Hello %s!\n", name)
+    },
+}
+
 func init() {
     rootCmd.AddCommand(versionCmd)
     rootCmd.AddCommand(serverCmd)
-    
+    rootCmd.AddCommand(greetCmd)
+
     // Add flags to server command
     serverCmd.Flags().String("config", "", "Config file path")
     serverCmd.Flags().String("env", "", "Environment (dev, staging, prod)")
@@ -97,6 +114,19 @@ func main() {
 }
 ```
 
+**Current Commands:**
+- `version`: Print version information
+- `server`: Start the DanceLessonsCoach server
+- `greet [name]`: Greet someone by name
+- `help`: Built-in help system
+- `completion`: Shell completion scripts (automatic)
+
+**Current Flags:**
+- `--config`: Config file path (server command)
+- `--env`: Environment (dev, staging, prod) (server command)
+- `--debug`: Enable debug logging (server command)
+- `--help`: Help for any command (built-in)
+
 #### Phase 2: Advanced Features (Future)
 - **Subcommand groups**: `server`, `db`, `migrate`, `tools`
 - **Persistent flags**: Global flags like `--config`, `--env`
@@ -170,10 +200,10 @@ dance-lessons-coach config validate
 - ✅ **Integration**: Works with existing config system
 
 **What's still needed?**
-- ❌ **Implementation**: Actual cobra integration
-- ❌ **Migration**: Move existing flags to cobra
-- ❌ **Documentation**: Update docs with new CLI
-- ❌ **Testing**: Ensure all functionality works
+- ✅ **Implementation**: Actual cobra integration (Phase 1 complete)
+- ❌ **Migration**: Move existing flags to cobra (Phase 2)
+- ❌ **Documentation**: Update docs with new CLI (Phase 2)
+- ❌ **Testing**: Ensure all functionality works (Phase 2)
 
 ### Future Enhancements