feat(issue): add assigned_by filter to list_issues

Add an optional assigned_by parameter to the list_issues tool that maps to
gitea_sdk.ListIssueOption.AssignedBy, and include assignees in the slim
list_issues output for consistency with the single-issue view.

Co-Authored-By: Codet <codet@commitgo.dev> (GPT-5-Codex)

operation/issue/slim.go

@@ -63,6 +63,9 @@ func slimIssues(issues []*gitea_sdk.Issue) []map[string]any {
 		if len(i.Labels) > 0 {
 			m["labels"] = slim.LabelNames(i.Labels)
 		}
+		if len(i.Assignees) > 0 {
+			m["assignees"] = slim.UserLogins(i.Assignees)
+		}
 		if i.Milestone != nil {
 			m["milestone"] = map[string]any{
 				"id":    i.Milestone.ID,