fix: prevent nil-panic on server and improve OAuth callback handling

Load config eagerly during server startup so sortByUrgency never
hits a nil config. Add nil-guard in BuildUrgencyCoefficients as
belt-and-suspenders defense. Fix OAuth callback to support both
GET and POST, and resolve issuer URLs properly with path.Dir.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

opal-task/cmd/server.go

@@ -113,6 +113,12 @@ Examples:
 			os.Exit(1)
 		}
 
+		// Load config (read-only — uses defaults if no opal.yml exists)
+		if _, err := engine.LoadConfig(); err != nil {
+			fmt.Fprintf(os.Stderr, "Error loading config: %v\n", err)
+			os.Exit(1)
+		}
+
 		// Initialize database
 		if err := engine.InitDB(); err != nil {
 			fmt.Fprintf(os.Stderr, "Error initializing database: %v\n", err)