feat: add interactive setup wizard for first-run configuration

Implement a comprehensive setup wizard to improve onboarding and
configuration experience for both personal and server deployments.

Key features:
- Interactive wizard with profile selection (personal/server/custom)
- Quick setup mode with sensible defaults
- First-run detection with helpful welcome message
- Directory configuration with validation
- Server OAuth/JWT configuration with auto-generation
- Environment file creation for server deployments
- Template generators for systemd service and env files

New commands:
- opal setup              # Interactive wizard
- opal setup --quick      # Quick setup with defaults
- opal setup --profile    # Use specific profile
- opal setup --show-systemd  # Show systemd template
- opal setup --show-env   # Show environment file template

Implementation:
- internal/wizard/prompts.go: Reusable prompt utilities
- internal/wizard/profiles.go: Profile definitions and templates
- cmd/setup.go: Main setup command implementation
- cmd/root.go: First-run detection and welcome message
- internal/engine/config.go: ConfigExists() and IsFirstRun() helpers

User experience:
- On first run, shows welcome message suggesting 'opal setup'
- Non-intrusive - creates defaults automatically if skipped
- Wizard guides through all configuration options
- Server setup includes OAuth/JWT configuration
- Environment file created with proper permissions (0600)
- Clear next steps displayed after completion

opal-task/internal/engine/config.go

@@ -219,6 +219,21 @@ func GetSyncConflictLogPath() (string, error) {
 	return filepath.Join(dataDir, "sync_conflicts.log"), nil
 }
 
+// ConfigExists checks if a configuration file exists
+func ConfigExists() bool {
+	configPath, err := GetConfigPath()
+	if err != nil {
+		return false
+	}
+	_, err = os.Stat(configPath)
+	return err == nil
+}
+
+// IsFirstRun checks if this is the first time opal is being run
+func IsFirstRun() bool {
+	return !ConfigExists()
+}
+
 // LoadConfig loads the configuration from file or creates default
 func LoadConfig() (*Config, error) {
 	if globalConfig != nil {