fix(printer): emit error and repo-list messages on stderr

PrintMsgErr and PrintRepoErr were writing to p.writer (stdout)
despite their names and despite the parallel Error method routing
its output to p.errWriter. That meant the gih CLI's error notices
("Failed: <repo>", "Timeout: <repo>") and the final failed/timed-out
repository list landed on stdout — indistinguishable from successful
results when the caller used `2>` to capture errors or `>` to
capture results.

Switch both methods to p.errWriter to match the Error method and the
implicit contract suggested by the *Err naming. Existing tests use
NewPrinter(&buf, &buf) so they keep passing; the new behaviour shows
up only when callers wire distinct writers, which is the use case
this fix enables.

Reported by GitHub Copilot on PR #26.

Co-Authored-By: Claude <noreply@anthropic.com>

Author: takecy

printer/print.go

@@ -114,27 +114,29 @@ func (p *Printer) PrintMsg(msg string) {
 	}
 }
 
-// PrintMsgErr prints error message
+// PrintMsgErr prints an error message to errWriter so that callers can
+// redirect stderr separately from stdout (consistent with Printer.Error).
 func (p *Printer) PrintMsgErr(msg string) {
 	type message struct {
 		Msg string
 	}
 	p.mu.Lock()
 	defer p.mu.Unlock()
-	if err := msgErrTpl.Execute(p.writer, message{Msg: msg}); err != nil {
+	if err := msgErrTpl.Execute(p.errWriter, message{Msg: msg}); err != nil {
 		log.Println(err)
 	}
 }
 
-// PrintRepoErr prints error message
+// PrintRepoErr prints a header plus a list of repository paths to errWriter
+// (matching the stderr semantics of PrintMsgErr / Error).
 func (p *Printer) PrintRepoErr(msg string, repos []string) {
 	type message struct {
 		Msg   string
 		Repos []string
 	}
 	p.mu.Lock()
 	defer p.mu.Unlock()
-	if err := repoErrTpl.Execute(p.writer, message{Msg: msg, Repos: repos}); err != nil {
+	if err := repoErrTpl.Execute(p.errWriter, message{Msg: msg, Repos: repos}); err != nil {
 		log.Println(err)
 	}
 }