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/custom.go

@@ -0,0 +1,14 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+// Package root registers custom business routes and frontend serving.
+package root
+
+import (
+	"github.com/gin-gonic/gin"
+)
+
+// RegisterCustomRootRoutes registers custom business routes that belong to the root path.
+func RegisterCustomRootRoutes(_ *gin.Engine) {
+	// Add custom root routes here
+}