Use sentinel error for nil command provider

Replace dynamic error with sentinel error ErrCommandNil for proper
error wrapping and type-safe error checking.

Changes:
- Import errors package in cmd/internal/registry.go
- Use fmt.Errorf with %w to wrap ErrCommandNil sentinel error
- Import errors package in registry_test.go
- Update test to use errors.Is() for sentinel error checking
- Verify error message includes provider name for debugging

This follows Atmos error handling conventions per CLAUDE.md:
- All errors wrapped using static errors from errors/errors.go
- Use errors.Is() for error checking
- Use fmt.Errorf with %w for adding context

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: osterman

cmd/internal/registry.go

@@ -5,6 +5,8 @@ import (
 	"sync"
 
 	"github.com/spf13/cobra"
+
+	errUtils "github.com/cloudposse/atmos/errors"
 )
 
 // registry is the global command registry instance.
@@ -74,7 +76,7 @@ func RegisterAll(root *cobra.Command) error {
 	for name, provider := range registry.providers {
 		cmd := provider.GetCommand()
 		if cmd == nil {
-			return fmt.Errorf("command provider %s returned nil command", name)
+			return fmt.Errorf("%w: provider %s", errUtils.ErrCommandNil, name)
 		}
 
 		root.AddCommand(cmd)

cmd/internal/registry_test.go

@@ -1,12 +1,15 @@
 package internal
 
 import (
+	"errors"
 	"fmt"
 	"testing"
 
 	"github.com/spf13/cobra"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
+
+	errUtils "github.com/cloudposse/atmos/errors"
 )
 
 // mockCommandProvider is a test implementation of CommandProvider.
@@ -154,7 +157,8 @@ func TestRegisterAllNilCommand(t *testing.T) {
 
 	err := RegisterAll(rootCmd)
 	assert.Error(t, err)
-	assert.Contains(t, err.Error(), "nil command")
+	assert.True(t, errors.Is(err, errUtils.ErrCommandNil), "error should wrap ErrCommandNil")
+	assert.Contains(t, err.Error(), "test", "error message should include provider name")
 }
 
 func TestGetProviderNotFound(t *testing.T) {