loop: add email address to Co-Authored-By trailer

- It's the standard.
- It'll let us set up a gravatar for hello@sketch.dev with the sketch logo.
- It tells people how to find sketch (there was no domain or other hint anywhere).
diff --git a/loop/donetool.go b/loop/donetool.go
index c02e5a5..e4b0542 100644
--- a/loop/donetool.go
+++ b/loop/donetool.go
@@ -41,7 +41,7 @@
 }
 
 func doneChecklistJSONSchema(gitUsername, gitEmail string) string {
-	gitCommitDescription := fmt.Sprintf(`Create git commits for any code changes you made. Match the style of recent commit messages. Include 'Co-Authored-By: sketch' and the original user prompt. Use GIT_AUTHOR_NAME="%s" GIT_AUTHOR_EMAIL="%s" (not git config).`,
+	gitCommitDescription := fmt.Sprintf(`Create git commits for any code changes you made. Match the style of recent commit messages. Include 'Co-Authored-By: sketch <hello@sketch.dev>' and the original user prompt. Use GIT_AUTHOR_NAME="%s" GIT_AUTHOR_EMAIL="%s" (not git config).`,
 		gitUsername, gitEmail)
 	desc, err := json.Marshal(gitCommitDescription)
 	if err != nil {