age -o keeps a symlink, pipe, device or redirected stream at the path (closes #59)

age encrypt -o and age decrypt -o now look at the -o path before writing. A path that is the same file as the tool's standard output or standard error, under any name, is written to that stream, so a redirected file keeps its contents, inode and mode. A new path or a regular file is written beside it and renamed over it, as before, with the signal handling of #48. A symlink gets the same rule for what it points at, so the link survives; a dangling one is refused. A named pipe or a device is written directly. The README says a replaced file gets mode 0600.

Rule suppressed: gosec G304 on the direct open of the -o path.

Model: opus-5-5

internal/cli/cli.go

@@ -85,7 +85,8 @@ func init() {
 // 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.
+// write a new file to rename over the named one, so the unfinished file
+// is removed.
 func Main() int {
 	err := Root().Execute()
 	if err == nil {