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/sync.go

@@ -286,13 +286,11 @@ var syncLogCmd = &cobra.Command{
 	Short: "Show conflict resolution log",
 	Long:  `Display the log of sync conflicts and how they were resolved`,
 	Run: func(cmd *cobra.Command, args []string) {
-		configDir, err := engine.GetConfigDir()
+		logPath, err := engine.GetSyncConflictLogPath()
 		if err != nil {
 			fmt.Fprintf(os.Stderr, "Error: %v\n", err)
 			os.Exit(1)
 		}
-
-		logPath := fmt.Sprintf("%s/sync_conflicts.log", configDir)
 		data, err := os.ReadFile(logPath)
 		if err != nil {
 			if os.IsNotExist(err) {