Signals end every command, not only ssh to and ssh install (closes #48)

SIGINT, SIGTERM and SIGHUP were caught for the whole run, so the
mnemonic prompt waited for Enter and an interrupted `age encrypt -o`
put the encryption of the cut-off input in place. Now they end the
tool at once, except where a command cleans up first: `ssh to` and
`ssh install` while ssh or sftp runs, and `age encrypt -o` and
`age decrypt -o` while they write. A signal those two have received
when their input ends removes the unfinished file and exits 1; a later
one leaves the whole file in place. The tool stays on the main thread,
where Linux delivers the signal first. Signals the tool was started
ignoring stay ignored, so a run under nohup survives a hangup.

Model: opus-5-5

internal/cli/cli.go

@@ -2,13 +2,11 @@
 package cli
 
 import (
-	"context"
 	"errors"
 	"fmt"
 	"os"
-	"os/signal"
+	"runtime"
 	"runtime/debug"
-	"syscall"
 
 	"github.com/spf13/cobra"
 	"sneak.berlin/go/keyfunc/internal/cli/age"
@@ -64,24 +62,32 @@ func Root() *cobra.Command {
 	return root
 }
 
+// init keeps the command on the main thread. Linux hands a signal sent
+// to the tool to that thread first, and a thread runs a pending signal
+// handler before its own code, so when "age encrypt -o" or "age
+// decrypt -o" checks for a signal as its input ends, one sent before
+// then, as by Ctrl-C on a pipeline, has been received.
+//
+//nolint:gochecknoinits // only an init can keep main on the main thread
+func init() {
+	runtime.LockOSThread()
+}
+
 // Main runs the tool and returns the status the process should exit
 // with. An error ends the tool with status 1, except when it carries a
 // status of its own, which "ssh to" uses to hand on the status ssh
 // ended with. ssh has already said whatever it had to say in that
 // case, so nothing more is printed.
 //
-// SIGINT, SIGTERM and SIGHUP cancel the command's context instead of
-// killing the process outright, so the child ssh or sftp ends and the
-// deferred cleanup that removes the agent socket and the install
-// working directory still runs.
+// SIGINT, SIGTERM and SIGHUP end the tool at once, as they end any Go
+// program, so a command waiting at the mnemonic prompt or reading what
+// it encrypts or decrypts goes no further. The exceptions catch the
+// signals to clean up first: "ssh to" and "ssh install" while they
+// have ssh or sftp running, so the child ends and their own cleanup
+// still runs, and "age encrypt -o" and "age decrypt -o" while they
+// write, so the unfinished file is removed.
 func Main() int {
-	ctx, stop := signal.NotifyContext(
-		context.Background(),
-		syscall.SIGINT, syscall.SIGTERM, syscall.SIGHUP,
-	)
-	defer stop()
-
-	err := Root().ExecuteContext(ctx)
+	err := Root().Execute()
 	if err == nil {
 		return 0
 	}