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/engine/database.go

@@ -3,6 +3,7 @@ package engine
 import (
 	"database/sql"
 	"fmt"
+	"os"
 
 	_ "github.com/mattn/go-sqlite3"
 )
@@ -20,6 +21,15 @@ func InitDB() error {
 		return fmt.Errorf("failed to get database path: %w", err)
 	}
 
+	// Ensure data directory exists
+	dataDir, err := GetDataDir()
+	if err != nil {
+		return fmt.Errorf("failed to get data directory: %w", err)
+	}
+	if err := os.MkdirAll(dataDir, 0755); err != nil {
+		return fmt.Errorf("failed to create data directory: %w", err)
+	}
+
 	// Open database connection
 	database, err := sql.Open("sqlite3", dbPath)
 	if err != nil {