fix: enroll 写出守护进程会拒绝的配置，install 写出与自身路径不符的单元

四处"校验在错误的层"的缺陷，都属于同一类：每个组件单独看都对，接缝错了。

1. `msa-agent enroll --relay http://relay.example` / `relay.example` /
   `https://relay.example/v1` 都会写出配置文件，install 报成功，守护进程在**下一次启动**才
   拒绝，日志里只有一行 "msa-agent exiting"。规则原本散在三处（config 只查非空、transport
   查真实规则、enroll 的注册客户端另有一份），现在收进 `internal/relayurl` 一份，三处都调它。
   `config.File.Validate` 因此也补上了 https/host/无路径/无 query 的检查。

2. `enroll` 不再自己手写字段检查，改为构造真正的 `config.File` 并调用真正的 `Validate`——
   一份"什么是可用配置"的定义，结构上无法再漂移。`config.Marshal` 让 enroll 与 Save 用同一个
   编码器。

3. `install --config /srv/msa/config.json` 会给该文件改属主、交给服务用户，然后写一个
   ExecStart 指向 `/etc/msa-agent/config.json` 的单元——服务起来找不到配置就退出，而它需要的
   文件就在别处、属主正确、从未被读。`enroll --audit /srv/audit/audit.log` 同理：单元的
   `ReadWritePaths` 只覆盖 `/var/log/msa-agent`，在 `ProtectSystem=strict` 下 agent 的
   `MkdirAll` 报 EROFS，进程启动即退出，报错既不提那个 flag 也不提那条指令。现在单元是模板，
   `RenderedUnit` 按实际布局渲染，`--print-unit` 打印的就是会被写入的那一份；默认布局渲染结果
   与仓库里被 review 过的文件逐字节相同，所以 vendored 副本的漂移检查仍然有意义。

测试：新增 `internal/relayurl`，`config`/`enroll`/`install` 各补回归测试。每条都做了变异验证
（把修复回退，对应测试必须失败）。

internal/enroll/register.go

@@ -9,10 +9,11 @@ import (
 	"fmt"
 	"io"
 	"net/http"
-	"net/url"
 	"os"
 	"strings"
 	"time"
+
+	"fun.chick/msa-agent/internal/relayurl"
 )
 
 // Register announces this node to the relay, using a one-time code an operator generated in the app.
@@ -43,14 +44,11 @@ type RegisterOptions struct {
 // The failure vocabulary is deliberately small and each case names the next action, because this runs
 // during an install and the operator is looking at one line of output.
 func Register(ctx context.Context, opts RegisterOptions) error {
-	parsed, err := url.Parse(opts.RelayURL)
-	if err != nil {
-		return fmt.Errorf("enroll: relay url: %w", err)
-	}
-	if parsed.Scheme != "https" {
-		// https is not negotiable here for the same reason the agent refuses it: a registration sent in
-		// the clear can be replayed by anyone who sees it, and the code is single-use.
-		return fmt.Errorf("enroll: relay url must be https, got %q", parsed.Scheme)
+	// The same rule the daemon applies, from the same package. This check is not redundant with the
+	// config validation in Generate: registration happens *before* the config is written, so a URL this
+	// rejected late would already have been sent to a host the operator did not mean to reach.
+	if err := relayurl.Validate(opts.RelayURL); err != nil {
+		return fmt.Errorf("enroll: %w", err)
 	}
 	if strings.TrimSpace(opts.Code) == "" {
 		return fmt.Errorf("enroll: the enrollment code is empty")