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/v1/v1.go

@@ -0,0 +1,24 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+// Package v1 contains router registrations for API V1
+package v1
+
+import (
+	"github.com/gin-gonic/gin"
+)
+
+// RegisterV1Routes registers all routes under API V1.
+func RegisterV1Routes(apiV1Router *gin.RouterGroup, apiGroup *gin.RouterGroup) {
+	// 1. Public (captcha, health, config)
+	RegisterPublicRoutes(apiV1Router, apiGroup)
+
+	// 2. OAuth, User, Upload
+	RegisterUserRoutes(apiV1Router)
+
+	// 3. Admin
+	RegisterAdminRoutes(apiV1Router)
+
+	// 4. Register custom business routes
+	RegisterCustomRoutes(apiV1Router)
+}