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>