fix(agent): 卸载时指出该去哪删节点，而不是一律说 agents.json

agent 不持有运维令牌——节点不是运维，一个能删掉邻居的被控节点比它解决的问题更糟——所以
它不能自己从中继上摘掉自己。此前卸载的结束语对所有节点都写"the relay's agents.json still
lists this node"，而自注册的节点根本不在那里：它在中继的状态文件里，要从手机上删。运维照着
那句话去翻一个没有这条记录的文件。

现在节点在登记时把来历写进配置（`selfEnrolled`，带 `--code` 为真），卸载结束语据此给出
正确的那一句：

- 自注册的 → "在中继的登记状态里，去 App 的「中继节点」移除"
- 手工登记的 → "在 agents.json 里，删掉那条并重启中继"
- 配置读不出来 → 两句都提，因为读不到不是让卸载失败的理由

测试两项：卸载打印的是它被告知的那一句（且不再对自注册节点说 agents.json）、登记时正确
记录来历（带码为真、不带码不写这个字段）。

Go 全包、Node 141 项、Swift 121 项通过。

internal/install/uninstall.go

@@ -81,6 +81,10 @@ type Options struct {
 	// NotifyRelay retires this node's device bearers at the relay, and is called before anything is
 	// removed. Nil skips it, which is right for a node that was never paired.
 	NotifyRelay func() error
+	// RemovalHint is the sentence printed at the end telling the operator what is left at the relay.
+	// It depends on how the node got there — self-enrolled or written into agents.json — and the caller
+	// is what knows that, from the config the node wrote at enrollment time.
+	RemovalHint string
 	Runner      Runner
 	Log         func(format string, args ...any)
 }
@@ -164,7 +168,10 @@ func Uninstall(opts Options) error {
 		opts.Log("dry run: nothing was removed")
 	} else {
 		opts.Log("msa-agent removed")
-		opts.Log("the relay's agents.json still lists this node; remove that entry unless you are reinstalling")
+		// The one thing this command cannot do for the operator, and it cannot by design: the agent holds
+		// no admin token, because a node is not an operator, and a compromised node that could remove its
+		// neighbours would be a worse problem than the one this hint solves.
+		opts.Log("%s", opts.RemovalHint)
 	}
 	return nil
 }