fix: update SetLastPRSync to use version date instead of current time

- Change last_pr_sync to use versionDate instead of time.Now()
- Ensure future runs fetch PRs merged after the version date
- Add clarifying comments explaining the sync timing logic

cmd/generate_changelog/internal/changelog/processing.go

@@ -291,8 +291,9 @@ func (g *Generator) CreateNewChangelogEntry(version string) error {
 			fmt.Fprintf(os.Stderr, "Warning: Failed to update last_processed_tag: %v\n", err)
 		}
 
-		// Update last_pr_sync to current time
-		if err := g.cache.SetLastPRSync(time.Now()); err != nil {
+		// Update last_pr_sync to the version date (not current time)
+		// This ensures future runs will fetch PRs merged after this version
+		if err := g.cache.SetLastPRSync(versionDate); err != nil {
 			fmt.Fprintf(os.Stderr, "Warning: Failed to update last_pr_sync: %v\n", err)
 		}
 	}