fix: match 'Last updated' timestamp with or without bold markers (#1836)

The template outputs plain text `Last updated: [DATE]` but both
update-agent-context scripts only matched `**Last updated**: [DATE]`
(bold Markdown). Make the bold markers optional in the regex so the
timestamp is refreshed regardless of formatting.

Co-authored-by: easonysliu <easonysliu@tencent.com>
Co-authored-by: Claude (claude-opus-4-6) <noreply@anthropic.com>

scripts/powershell/update-agent-context.ps1

@@ -331,7 +331,7 @@ function Update-ExistingAgentFile {
             if ($existingChanges -lt 2) { $output.Add($line); $existingChanges++ }
             continue
         }
-        if ($line -match '\*\*Last updated\*\*: .*\d{4}-\d{2}-\d{2}') {
+        if ($line -match '(\*\*)?Last updated(\*\*)?: .*\d{4}-\d{2}-\d{2}') {
             $output.Add(($line -replace '\d{4}-\d{2}-\d{2}',$Date.ToString('yyyy-MM-dd')))
             continue
         }