Port the CLI to Go: awg-control <start|stop|…>

internal/cli reimplements cli/src: start/stop/restart/status with the
awg-ctrl health gate, credentials, join/unjoin, peers [drop], monitor
install/status/remove and the interactive menu. Same cli.env, pid files,
logs and messages. Without arguments or with an operator command the binary
acts as the CLI; ctrl/ui/agent stay service roles.

- services are started as the same binary (`awg-control ctrl|ui|agent`),
  detached into their own session, from the same working directories
- processes started by the Node CLI (tsx … index.ts / server.ts) are still
  recognised, so `stop` and `status` work across the switch
- after join the fresh restart gets the original environment, so a rejoin
  picks up the new cli.env instead of the inherited old CORE_* values
- install root: AWG_ROOT, else the nearest parent of the binary with cli/ or
  .env, else /opt/awg-control

Checked end to end on stubbed awg: start/status/stop, credentials, join to
the panel's own hub (with and without --no-panel), peers listing and drop,
unjoin, menu.

internal/cli/cli_test.go

@@ -0,0 +1,44 @@
+package cli
+
+import "testing"
+
+func TestUpdateEnvVar(t *testing.T) {
+	for _, c := range []struct{ in, key, val, want string }{
+		{"", "A", "1", "\nA=1\n"},
+		{"A=0\nB=2\n", "A", "1", "A=1\nB=2\n"},
+		{"B=2\n\n\n", "A", "1", "B=2\nA=1\n"},
+		{"XA=0\nA=0\n", "A", "$&1", "XA=0\nA=$&1\n"},
+		{"A=0\nA=9\n", "A", "1", "A=1\nA=9\n"}, // как String.replace без g — только первое
+	} {
+		if got := updateEnvVar(c.in, c.key, c.val); got != c.want {
+			t.Errorf("updateEnvVar(%q, %s, %s) = %q, want %q", c.in, c.key, c.val, got, c.want)
+		}
+	}
+	if got := removeEnvVar("A=1\nJOIN_SECRET=x\nB=2\n", "JOIN_SECRET"); got != "A=1\nB=2\n" {
+		t.Errorf("removeEnvVar: %q", got)
+	}
+}
+
+func TestAreaOf(t *testing.T) {
+	for owner, want := range map[string]string{
+		"abcdef123456/m1/d2": "abcdef123456",
+		"m1/d2":              "legacy",
+		"alice":              "",
+		"ABCDEF123456/m1/d2": "",
+		"abc/m1/d2":          "",
+		"":                   "",
+	} {
+		if got := areaOf(owner); got != want {
+			t.Errorf("areaOf(%q) = %q, want %q", owner, got, want)
+		}
+	}
+}
+
+func TestPeersWord(t *testing.T) {
+	for n, want := range map[int]string{0: "пиров", 1: "пир", 2: "пира", 4: "пира", 5: "пиров", 11: "пиров", 12: "пиров",
+		21: "пир", 22: "пира", 101: "пир", 111: "пиров", 114: "пиров", 1004: "пира"} {
+		if got := peersWord(n); got != want {
+			t.Errorf("peersWord(%d) = %s, want %s", n, got, want)
+		}
+	}
+}