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/enroll/enroll_test.go

@@ -142,3 +142,27 @@ func TestMissingInputIsRefused(t *testing.T) {
 		}
 	}
 }
+
+// The node records how it got into the relay, because `uninstall` cannot work it out any other way: the
+// agent holds no admin token, so it cannot remove its own entry, and the operator has to be told which
+// of the relay's two records mentions this node.
+func TestEnrollmentRecordsItsOwnProvenance(t *testing.T) {
+	withCode, err := Generate(Options{
+		RelayURL: "https://relay.example", LocalPort: 3000, AuditPath: "/a", SelfEnrolled: true,
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !strings.Contains(string(withCode.Config), `"selfEnrolled": true`) {
+		t.Fatal("a self-enrolled node does not say so")
+	}
+	// And a node an operator wrote into agents.json does not claim to have enrolled itself, or the
+	// uninstall would send the operator to the app to remove an entry that is not there.
+	manual, err := Generate(Options{RelayURL: "https://relay.example", LocalPort: 3000, AuditPath: "/a"})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if strings.Contains(string(manual.Config), "selfEnrolled") {
+		t.Fatal("a manually registered node claims to be self-enrolled")
+	}
+}