Add description generation for existing pull requests (#8254)

* Initial plan

* Initial exploration of codebase

Co-authored-by: alexr00 <38270282+alexr00@users.noreply.github.com>

* Add description generation for existing PRs with sparkle icon UI

Co-authored-by: alexr00 <38270282+alexr00@users.noreply.github.com>

* Fix: Populate patches array with actual PR diff data for better description generation

Co-authored-by: alexr00 <38270282+alexr00@users.noreply.github.com>

* Improve useEffect to properly handle state changes

Co-authored-by: alexr00 <38270282+alexr00@users.noreply.github.com>

* Clean up

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: alexr00 <38270282+alexr00@users.noreply.github.com>

Author: Copilot

common/views.ts

@@ -166,6 +166,10 @@ export interface TitleAndDescriptionResult {
 	description: string | undefined;
 }
 
+export interface DescriptionResult {
+	description: string | undefined;
+}
+
 export interface CloseResult {
 	state: GithubItemStateEnum;
 	commentEvent?: CommentEvent;