Remove os.Exit() from library code (gt-fm75)

Refactor to return errors instead of calling os.Exit() directly:
- Add SilentExitError type for commands that signal status via exit code
- Update mail.go runMailPeek() and runMailCheck() to return errors
- Change Execute() to return int exit code instead of calling os.Exit()
- Move os.Exit() call to main() where it belongs

This improves testability, enables graceful shutdown, and follows Go
conventions for library code.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

cmd/gt/main.go

@@ -1,8 +1,12 @@
 // gt is the Gas Town CLI for managing multi-agent workspaces.
 package main
 
-import "github.com/steveyegge/gastown/internal/cmd"
+import (
+	"os"
+
+	"github.com/steveyegge/gastown/internal/cmd"
+)
 
 func main() {
-	cmd.Execute()
+	os.Exit(cmd.Execute())
 }

internal/cmd/errors.go

@@ -0,0 +1,31 @@
+package cmd
+
+import "fmt"
+
+// SilentExitError signals that the command should exit with a specific code
+// without printing an error message. This is used for scripting purposes
+// where exit codes convey status (e.g., "no mail" = exit 1).
+type SilentExitError struct {
+	Code int
+}
+
+func (e *SilentExitError) Error() string {
+	return fmt.Sprintf("exit %d", e.Code)
+}
+
+// NewSilentExit creates a SilentExitError with the given exit code.
+func NewSilentExit(code int) *SilentExitError {
+	return &SilentExitError{Code: code}
+}
+
+// IsSilentExit checks if an error is a SilentExitError and returns its code.
+// Returns 0 and false if err is nil or not a SilentExitError.
+func IsSilentExit(err error) (int, bool) {
+	if err == nil {
+		return 0, false
+	}
+	if se, ok := err.(*SilentExitError); ok {
+		return se.Code, true
+	}
+	return 0, false
+}

internal/cmd/mail.go

@@ -579,23 +579,20 @@ func runMailPeek(cmd *cobra.Command, args []string) error {
 	// All mail uses town beads (two-level architecture)
 	workDir, err := findMailWorkDir()
 	if err != nil {
-		os.Exit(1) // Silent exit - no workspace
-		return nil
+		return NewSilentExit(1) // Silent exit - no workspace
 	}
 
 	// Get mailbox
 	router := mail.NewRouter(workDir)
 	mailbox, err := router.GetMailbox(address)
 	if err != nil {
-		os.Exit(1) // Silent exit - can't access mailbox
-		return nil
+		return NewSilentExit(1) // Silent exit - can't access mailbox
 	}
 
 	// Get unread messages
 	messages, err := mailbox.ListUnread()
 	if err != nil || len(messages) == 0 {
-		os.Exit(1) // Silent exit - no unread
-		return nil
+		return NewSilentExit(1) // Silent exit - no unread
 	}
 
 	// Show first unread message
@@ -923,12 +920,10 @@ func runMailCheck(cmd *cobra.Command, args []string) error {
 	// Normal mode
 	if unread > 0 {
 		fmt.Printf("%s %d unread message(s)\n", style.Bold.Render("📬"), unread)
-		os.Exit(0)
-	} else {
-		fmt.Println("No new mail")
-		os.Exit(1)
+		return NewSilentExit(0)
 	}
-	return nil
+	fmt.Println("No new mail")
+	return NewSilentExit(1)
 }
 
 func runMailThread(cmd *cobra.Command, args []string) error {

internal/cmd/root.go

@@ -2,7 +2,6 @@
 package cmd
 
 import (
-	"os"
 	"strings"
 
 	"github.com/spf13/cobra"
@@ -29,11 +28,18 @@ across distributed teams of AI agents working on shared codebases.`,
 	},
 }
 
-// Execute runs the root command
-func Execute() {
+// Execute runs the root command and returns an exit code.
+// The caller (main) should call os.Exit with this code.
+func Execute() int {
 	if err := rootCmd.Execute(); err != nil {
-		os.Exit(1)
+		// Check for silent exit (scripting commands that signal status via exit code)
+		if code, ok := IsSilentExit(err); ok {
+			return code
+		}
+		// Other errors already printed by cobra
+		return 1
 	}
+	return 0
 }
 
 // Command group IDs - used by subcommands to organize help output