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/internal/sync/queue.go

@@ -4,7 +4,6 @@ import (
 	"encoding/json"
 	"fmt"
 	"os"
-	"path/filepath"
 
 	"git.jnss.me/joakim/opal/internal/engine"
 	"github.com/google/uuid"
@@ -27,12 +26,11 @@ type Queue struct {
 
 // NewQueue creates a new queue instance
 func NewQueue() (*Queue, error) {
-	configDir, err := engine.GetConfigDir()
+	queuePath, err := engine.GetSyncQueuePath()
 	if err != nil {
 		return nil, err
 	}
 
-	queuePath := filepath.Join(configDir, "sync_queue.json")
 	q := &Queue{filepath: queuePath}
 
 	if err := q.load(); err != nil {