fix(amend): default author falls back to USERNAME on Windows (TP-056)

The env fallback chain only checked $USER, which Windows does not set —
docops amend without --by errored on any Windows box lacking git
user.name. Unmasked by the Windows CI un-hang: TestLifecycle_EndToEnd.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: nachiket

internal/amender/amender.go

@@ -98,7 +98,7 @@ func Run(opts Options) (Result, error) {
 		by = defaultAuthor()
 	}
 	if by == "" {
-		return Result{}, errors.New("amender: --by is required (no $DOCOPS_USER, git user.name, or $USER set)")
+		return Result{}, errors.New("amender: --by is required (no $DOCOPS_USER, git user.name, $USER, or $USERNAME set)")
 	}
 
 	date := opts.Date
@@ -427,7 +427,7 @@ func atomicWrite(path string, data []byte, mode os.FileMode) error {
 	return nil
 }
 
-// defaultAuthor consults DOCOPS_USER → git user.name → USER.
+// defaultAuthor consults DOCOPS_USER → git user.name → USER/USERNAME.
 // Returns "" when none is set; caller emits a clear error.
 func defaultAuthor() string {
 	if v := os.Getenv("DOCOPS_USER"); v != "" {
@@ -439,6 +439,10 @@ func defaultAuthor() string {
 	if v := os.Getenv("USER"); v != "" {
 		return v
 	}
+	// Windows spells it USERNAME.
+	if v := os.Getenv("USERNAME"); v != "" {
+		return v
+	}
 	return ""
 }