Add agentpr pr edit subcommand

Update a PR's title and/or body as the agent user, sending only the
fields supplied.

cmd/agentpr/main_test.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"io"
+	"strings"
 	"testing"
 )
 
@@ -16,3 +17,19 @@ func TestExecuteBadRepoErrors(t *testing.T) {
 		t.Fatal("Execute() = nil, want error for a malformed --repo")
 	}
 }
+
+// `pr edit` with neither --title nor --body has nothing to send; it must fail
+// with a usage error before any Vault/Gitea call, so this stays hermetic.
+func TestPREditRequiresTitleOrBody(t *testing.T) {
+	cmd := newRootCmd()
+	cmd.SetArgs([]string{"pr", "edit", "--repo", "unkin/repo", "--pr", "7"})
+	cmd.SetOut(io.Discard)
+	cmd.SetErr(io.Discard)
+	err := cmd.Execute()
+	if err == nil {
+		t.Fatal("Execute() = nil, want an error when neither --title nor --body is given")
+	}
+	if !strings.Contains(err.Error(), "--title or --body") {
+		t.Errorf("error = %q, want it to name the missing flags", err)
+	}
+}