refactor(router): organize root routes and centralize v1 route registration

- Created `internal/router/root` package to register root-level paths
- Moved files serving, robots.txt, and Swagger docs to `root/default.go`
- Registered custom root routes under `root/custom.go`
- Centralized all v1 routes registration in `internal/router/v1/v1.go`
- Simplified `internal/router/router.go` by delegating route registration
- Updated `new-api` skill `SKILL.md` to document the new router structure

internal/router/root/default.go

@@ -0,0 +1,28 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package root
+
+import (
+	_ "github.com/Rain-kl/Wavelet/docs" // Swagger documentation generation setup
+	publicconfig "github.com/Rain-kl/Wavelet/internal/apps/config"
+	"github.com/Rain-kl/Wavelet/internal/apps/upload"
+	"github.com/Rain-kl/Wavelet/internal/config"
+	"github.com/gin-gonic/gin"
+	swaggerFiles "github.com/swaggo/files"
+	ginSwagger "github.com/swaggo/gin-swagger"
+)
+
+// RegisterDefaultRootRoutes registers default routes that belong to the root path.
+func RegisterDefaultRootRoutes(r *gin.Engine) {
+	// 1. Serve files by ID
+	r.GET("/f/:id", upload.ServeFileByID)
+
+	// 2. Dynamic robots.txt serving
+	r.GET("/robots.txt", publicconfig.GetRobotsTXT)
+
+	// 3. Swagger routes (Non-production only)
+	if !config.Config.App.IsProduction() {
+		r.GET(config.Config.App.APIPrefix+"/swagger/*any", ginSwagger.WrapHandler(swaggerFiles.Handler))
+	}
+}