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/down.go

@@ -6,6 +6,7 @@ package cli
 
 import (
 	"context"
+	"flag"
 	"fmt"
 
 	"github.com/peterbourgon/ff/v3/ffcli"
@@ -17,7 +18,14 @@ var downCmd = &ffcli.Command{
 	ShortUsage: "down",
 	ShortHelp:  "Disconnect from Tailscale",
 
-	Exec: runDown,
+	Exec:    runDown,
+	FlagSet: newDownFlagSet(),
+}
+
+func newDownFlagSet() *flag.FlagSet {
+	downf := newFlagSet("down")
+	registerAcceptRiskFlag(downf)
+	return downf
 }
 
 func runDown(ctx context.Context, args []string) error {
@@ -25,6 +33,12 @@ func runDown(ctx context.Context, args []string) error {
 		return fmt.Errorf("too many non-flag arguments: %q", args)
 	}
 
+	if isSSHOverTailscale() {
+		if err := presentRiskToUser(riskLoseSSH, `You are connected over Tailscale; this action will disable Tailscale and result in your session disconnecting.`); err != nil {
+			return err
+		}
+	}
+
 	st, err := localClient.Status(ctx)
 	if err != nil {
 		return fmt.Errorf("error fetching current status: %w", err)