refactor: implement configurable directory structure with XDG support

Separate configuration from data storage and make paths configurable
via environment variables and command-line flags. This improves
Unix/Linux compliance and supports both development and production
deployments.

Key changes:
- Separate config dir (opal.yml) from data dir (database, logs)
- Support XDG Base Directory specification
- Add --config-dir and --data-dir flags
- Environment variables: OPAL_CONFIG_DIR, OPAL_DATA_DIR, OPAL_DB_PATH
- Smart fallback: /etc/opal, /var/lib/opal -> ~/.config/opal, ~/.local/share/opal
- Server mode validates required OAuth/JWT environment variables
- Update naming from 'jade' to 'opal' throughout
- Update systemd service name to 'opal.service'
- Add migration guide in README

Default paths:
- Config: /etc/opal (fallback: ~/.config/opal)
- Data: /var/lib/opal (fallback: ~/.local/share/opal)

Files modified:
- internal/engine/config.go: New directory resolution logic
- internal/engine/database.go: Auto-create data directory
- cmd/root.go: Add global flags for directory overrides
- cmd/server.go: Add configuration validation
- cmd/sync.go, internal/sync/*: Use new path helper functions
- tests: Update to use directory overrides
- docs: Update deployment guide and README

opal-task/cmd/root.go

@@ -21,6 +21,12 @@ type contextKey string
 
 const parsedArgsKey contextKey = "parsedArgs"
 
+// Global flags
+var (
+	configDirFlag string
+	dataDirFlag   string
+)
+
 // Command classification
 var commandNames = []string{
 	"add", "done", "modify", "delete",
@@ -188,6 +194,12 @@ func preprocessArgs(args []string) *ParsedArgs {
 }
 
 func init() {
+	// Add persistent flags for directory overrides
+	rootCmd.PersistentFlags().StringVar(&configDirFlag, "config-dir", "",
+		"Config directory (default: $XDG_CONFIG_HOME/opal or ~/.config/opal)")
+	rootCmd.PersistentFlags().StringVar(&dataDirFlag, "data-dir", "",
+		"Data directory (default: $XDG_DATA_HOME/opal or ~/.local/share/opal)")
+
 	cobra.OnInitialize(initializeApp)
 
 	// Add regular subcommands
@@ -212,6 +224,14 @@ func init() {
 }
 
 func initializeApp() {
+	// Set directory overrides from flags if provided
+	if configDirFlag != "" {
+		engine.SetConfigDirOverride(configDirFlag)
+	}
+	if dataDirFlag != "" {
+		engine.SetDataDirOverride(dataDirFlag)
+	}
+
 	// Initialize database
 	if err := engine.InitDB(); err != nil {
 		fmt.Fprintf(os.Stderr, "Error initializing database: %v\n", err)