fix: redirect spinner output from stdout to stderr (#176)

The spinner goroutine was writing carriage-return + status text to
os.Stdout every 100ms. In the non-interactive TTY path, the child
process also writes directly to os.Stdout, causing both writers to
race on the same file descriptor. The \r emitted by the spinner
resets the cursor to column 0 mid-line, corrupting and truncating
the child process output.

Fix by writing all spinner/status output to os.Stderr, which is the
standard Unix convention for diagnostic and status messages. This is
also consistent with how progress.go and ShowWarning already behave.

https://claude.ai/code/session_012jMiRSS4Jx9Bs7a2F4S6KN

Co-authored-by: Claude <noreply@anthropic.com>

internal/ui/spinner.go

@@ -2,6 +2,7 @@ package ui
 
 import (
 	"fmt"
+	"os"
 	"sync"
 	"time"
 )
@@ -44,7 +45,7 @@ func StartSpinnerWithColor(msg string, c ColorFn) {
 				ticker.Stop()
 				return
 			case <-ticker.C:
-				fmt.Printf("\r%s ... %s", c("PMG: "+msg), string(frames[pos%length]))
+				fmt.Fprintf(os.Stderr, "\r%s ... %s", c("PMG: "+msg), string(frames[pos%length]))
 				pos += 1
 			}
 		}
@@ -69,6 +70,6 @@ func StopSpinner() {
 
 	spinnerChan = nil
 
-	fmt.Printf("\r")
-	fmt.Println()
+	fmt.Fprint(os.Stderr, "\r")
+	fmt.Fprintln(os.Stderr)
 }

internal/ui/ui.go

@@ -50,7 +50,7 @@ func SetVerbosityLevel(level VerbosityLevel) {
 
 func ClearStatus() {
 	StopSpinner()
-	fmt.Print("\r")
+	fmt.Fprint(os.Stderr, "\r")
 }
 
 func Block(config *BlockConfig) error {