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/task_test.go

@@ -9,12 +9,15 @@ import (
 )
 
 func TestMain(m *testing.M) {
-	// Setup test database
-	os.Setenv("HOME", "/tmp/opal-test")
+	// Setup test database with explicit directory overrides
+	testDir := "/tmp/opal-test"
 
-	// Ensure config directory exists
-	configDir := "/tmp/opal-test/.config/jade"
-	if err := os.MkdirAll(configDir, 0755); err != nil {
+	// Set directory overrides to use test directory
+	SetConfigDirOverride(testDir)
+	SetDataDirOverride(testDir)
+
+	// Ensure test directory exists
+	if err := os.MkdirAll(testDir, 0755); err != nil {
 		panic(err)
 	}
 
@@ -27,7 +30,7 @@ func TestMain(m *testing.M) {
 	code := m.Run()
 
 	// Cleanup
-	os.RemoveAll("/tmp/opal-test/.config")
+	os.RemoveAll(testDir)
 	os.Exit(code)
 }