cmd/tailscale/cli: add lose-ssh risk

This makes it so that the user is notified that the action
they are about to take may result in them getting disconnected from
the machine. It then waits for 5s for the user to maybe Ctrl+C out of
it.

It also introduces a `--accept-risk=lose-ssh` flag for automation, which
allows the caller to pre-acknowledge the risk.

The two actions that cause this are:
- updating `--ssh` from `true` to `false`
- running `tailscale down`

Updates #3802

Signed-off-by: Maisem Ali <maisem@tailscale.com>

cmd/tailscale/cli/ssh.go

@@ -21,6 +21,7 @@ import (
 	"inet.af/netaddr"
 	"tailscale.com/envknob"
 	"tailscale.com/ipn/ipnstate"
+	"tailscale.com/net/tsaddr"
 )
 
 var sshCmd = &ffcli.Command{
@@ -179,3 +180,28 @@ func nodeDNSNameFromArg(st *ipnstate.Status, arg string) (dnsName string, ok boo
 	}
 	return "", false
 }
+
+// getSSHClientEnvVar returns the "SSH_CLIENT" environment variable
+// for the current process group, if any.
+var getSSHClientEnvVar = func() string {
+	return ""
+}
+
+// isSSHOverTailscale checks if the invocation is in a SSH session over Tailscale.
+// It is used to detect if the user is about to take an action that might result in them
+// disconnecting from the machine (e.g. disabling SSH)
+func isSSHOverTailscale() bool {
+	sshClient := getSSHClientEnvVar()
+	if sshClient == "" {
+		return false
+	}
+	ipStr, _, ok := strings.Cut(sshClient, " ")
+	if !ok {
+		return false
+	}
+	ip, err := netaddr.ParseIP(ipStr)
+	if err != nil {
+		return false
+	}
+	return tsaddr.IsTailscaleIP(ip)
+}