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>

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
+}