fix(install): 刚装完的服务会失败，单元要说清楚而不是每 5 秒重启一次

我在真内核上把安装序列完整走了一遍（容器里补了 useradd/systemctl 的替身，其余全是真的），
暴露出一个只有真跑才看得见的缺陷：

装完 → `install` 启动服务 → 但那时**还没有配对的手机**，而守护进程**故意**拒绝空的设备列表
（一个什么都不能授权的 agent 不该坐在那里显得健康）。于是 `Restart=always` + `RestartSec=5`
变成每 5 秒重启一次，永远，journal 里刷同一行。实测：exit 1，"devices is empty, so no phone
could ever be authorised"。

配对发生在安装**之后**（运维在节点上跑 `--pair`，然后在手机上配对），所以这不是异常状态，
而是流程的必经一步。修法两条：

- 单元加 `StartLimitIntervalSec=300` / `StartLimitBurst=10`：十次之后 systemd 放弃，单元进入
  `failed`——一个运维看得见、能处理的状态，而不是无限重启。
- `install` 的结束语改成说清这一步：服务会退出并停住，下一步是 `--pair`，用
  `systemctl status` 看状态。此前它说"installed and running"，而那是假的。

同一轮演练确认了此前所有修复在真内核上成立：服务用户建了、配置与日志目录交给它了、配置对
该用户可读、单元来自二进制且内容一致、卸载按序拆干净（用户/目录/单元全无残留）。

Go 全包通过（新增一项守着"单元必须有启动上限，且 install 必须说出下一步"）。

README.md

@@ -39,6 +39,20 @@ relay's `agents.json` by hand. Without it, `enroll` prints the entry to add.
 Removing it again is `sudo msa-agent uninstall`; `--dry-run` shows the plan first, and `--keep-logs`
 keeps the audit trail.
 
+### After installing, pair
+
+The daemon refuses to start with no paired devices, on purpose: an agent that could authorise nothing
+must not sit there looking healthy. Pairing happens after install, so a freshly installed node's service
+will fail and stop — the unit's start limit makes systemd give up rather than restart every five seconds
+forever, and `systemctl status msa-agent` shows it as failed rather than flapping.
+
+```sh
+sudo /opt/msa-agent/msa-agent --pair     # opens the LAN window; then pair from the app
+```
+
+Then `systemctl start msa-agent` (or just wait — it is enabled) and check `journalctl -u msa-agent -n 20`
+for the policy digest and the device count.
+
 ## Subcommands
 
 | Command | What it does |
@@ -49,6 +63,7 @@ keeps the audit trail.
 | `uninstall` | Tells the relay first, then removes the service, unit, config, logs and user |
 | `version` | Prints the version stamped into the binary |
 | `--pair` | Opens the temporary LAN window a phone uses to pair; closes itself |
+| `--pair-bind` | The address that window binds to (an IP literal; the default is all interfaces) |
 | `--check` | Validates the config and policy, then exits |
 | `--once` | Runs a single poll cycle and exits |
 

internal/install/msa-agent.service

@@ -11,6 +11,17 @@ Restart=always
 # Backing off on our own is not enough: a relay that is down for an hour should not be hammered by
 # every node in the fleet the moment it comes back.
 RestartSec=5
+# A freshly installed node has no paired devices, and the daemon refuses to run in that state on
+# purpose: an agent that could authorise nothing must not sit there looking healthy. But pairing happens
+# *after* install — the operator runs `--pair` on the node and then pairs the phone — so between those
+# two moments this unit would restart every five seconds, forever, filling the journal with the same
+# line. Measured on a real kernel: exit 1, "devices is empty, so no phone could ever be authorised".
+#
+# StartLimitIntervalSec/StartLimitBurst make systemd give up instead: ten attempts inside five minutes
+# and the unit enters `failed`, which is a state an operator can see and act on. The install command
+# prints the next step precisely so that state is expected rather than surprising.
+StartLimitIntervalSec=300
+StartLimitBurst=10
 TimeoutStopSec=20
 KillSignal=SIGTERM
 

internal/install/service.go

@@ -141,9 +141,13 @@ func Install(opts InstallOptions) error {
 		opts.Log("dry run: nothing was changed")
 		return nil
 	}
-	opts.Log("msa-agent installed and running")
+	// The service will fail until a phone is paired, and that is expected rather than a fault: the daemon
-	opts.Log("check it with: journalctl -u %s -n 20", paths.ServiceName)
+	// refuses an empty device list so it cannot sit there looking healthy while authorising nothing. The
-	opts.Log("next: run `msa-agent --pair` once, then pair the phone from the app")
+	// unit's start limit turns that into a `failed` state instead of a restart loop, and this line is
+	// what makes the state expected.
+	opts.Log("msa-agent installed and enabled")
+	opts.Log("next: run `msa-agent --pair` once and pair the phone; until then the service exits and stops")
+	opts.Log("check it with: systemctl status %s; journalctl -u %s -n 20", paths.ServiceName, paths.ServiceName)
 	return nil
 }
 

internal/install/uninstall_test.go

@@ -426,3 +426,51 @@ func TestTheRemovalHintIsTheOneItWasGiven(t *testing.T) {
 		t.Fatalf("the uninstall still claims every node is in agents.json:\n%s", output)
 	}
 }
+
+// The install ends by starting a service that cannot run yet, and the unit has to say what to do about
+// that. A freshly installed node has no paired devices — pairing happens after install — and the daemon
+// refuses to run in that state on purpose, because an agent that could authorise nothing must not look
+// healthy. With `Restart=always` and `RestartSec=5` and no start limit, that is a restart every five
+// seconds until a phone pairs.
+//
+// Found by rehearsing the install on a real kernel: exit 1, "devices is empty, so no phone could ever be
+// authorised", and the journal filling with the same line.
+func TestTheUnitGivesUpInsteadOfRestartingForever(t *testing.T) {
+	unit := string(UnitFile())
+	if !strings.Contains(unit, "Restart=always") {
+		t.Skip("the unit no longer restarts, so there is nothing to bound")
+	}
+	if !strings.Contains(unit, "StartLimitIntervalSec=") || !strings.Contains(unit, "StartLimitBurst=") {
+		t.Fatal("the unit restarts without a start limit: an unpaired node would restart every RestartSec forever")
+	}
+	// And the install tells the operator what to do next, so the failed state is expected rather than
+	// surprising.
+	var lines []string
+	root := t.TempDir()
+	paths := Paths{
+		ServiceName: "msa-agent",
+		UnitPath:    filepath.Join(root, "etc/systemd/system/msa-agent.service"),
+		ConfigPath:  filepath.Join(root, "etc/msa-agent/config.json"),
+		LogDir:      filepath.Join(root, "var/log/msa-agent"),
+		BinaryPath:  filepath.Join(root, "opt/msa-agent/msa-agent"),
+		User:        "msa-agent",
+	}
+	for _, path := range []string{paths.BinaryPath, paths.ConfigPath} {
+		if err := os.MkdirAll(filepath.Dir(path), 0o755); err != nil {
+			t.Fatal(err)
+		}
+		if err := os.WriteFile(path, []byte("x"), 0o600); err != nil {
+			t.Fatal(err)
+		}
+	}
+	if err := Install(InstallOptions{
+		Paths:  paths,
+		Runner: &recorder{failOn: "getent"},
+		Log:    func(format string, args ...any) { lines = append(lines, fmt.Sprintf(format, args...)) },
+	}); err != nil {
+		t.Fatalf("Install: %v", err)
+	}
+	if !strings.Contains(strings.Join(lines, "\n"), "--pair") {
+		t.Fatalf("the install does not say to pair next:\n%s", strings.Join(lines, "\n"))
+	}
+}

systemd/msa-agent.service

@@ -11,6 +11,17 @@ Restart=always
 # Backing off on our own is not enough: a relay that is down for an hour should not be hammered by
 # every node in the fleet the moment it comes back.
 RestartSec=5
+# A freshly installed node has no paired devices, and the daemon refuses to run in that state on
+# purpose: an agent that could authorise nothing must not sit there looking healthy. But pairing happens
+# *after* install — the operator runs `--pair` on the node and then pairs the phone — so between those
+# two moments this unit would restart every five seconds, forever, filling the journal with the same
+# line. Measured on a real kernel: exit 1, "devices is empty, so no phone could ever be authorised".
+#
+# StartLimitIntervalSec/StartLimitBurst make systemd give up instead: ten attempts inside five minutes
+# and the unit enters `failed`, which is a state an operator can see and act on. The install command
+# prints the next step precisely so that state is expected rather than surprising.
+StartLimitIntervalSec=300
+StartLimitBurst=10
 TimeoutStopSec=20
 KillSignal=SIGTERM