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/strategy.go

@@ -3,7 +3,6 @@ package sync
 import (
 	"fmt"
 	"os"
-	"path/filepath"
 	"time"
 
 	"git.jnss.me/joakim/opal/internal/engine"
@@ -112,13 +111,11 @@ func resolveConflict(local, remote *engine.Task, strategy ConflictResolution) *e
 
 // logConflict writes conflict information to log file
 func logConflict(local, remote *engine.Task, winner *engine.Task) {
-	configDir, err := engine.GetConfigDir()
+	logPath, err := engine.GetSyncConflictLogPath()
 	if err != nil {
 		return
 	}
 
-	logPath := filepath.Join(configDir, "sync_conflicts.log")
-
 	winnerLabel := "local"
 	if winner.UUID == remote.UUID && winner.Modified.Equal(remote.Modified) {
 		winnerLabel = "remote"