Promote [Unreleased] to [1.9.0] for release

User-facing feature added: PR auto-refresh (commit 253afd1). Minor
SemVer bump per AGENTS.md section 12.

AI-Local-Session: 6df9761f-b968-469a-87e4-376987d4353a
AI-Cloud-Session: 4080c61e-0c31-4ba9-9c66-2bbaf6fb6bf6
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: Geeven Singh

CHANGELOG.md

@@ -12,6 +12,8 @@ body. Keep section headings exact and write notes in Markdown.
 
 ## [Unreleased]
 
+## [1.9.0] - 2026-06-06
+
 ### Added
 
 - **Pull-request auto-refresh.** When DiffViewer is showing a GitHub