fix(security): bump astro to 6.4.7 to clear GHSA-2pvr-wf23-7pc7, GHSA-8hv8-536x-4wqp, GHSA-jrpj-wcv7-9fh9 (aws-samples#356)

osv-scanner flagged astro 6.1.10 (docs site) for three advisories
(2 High, 1 Medium); all fixed in >=6.4.6. Bump to 6.4.7 (latest 6.x),
which satisfies @astrojs/starlight's astro ^6.0.0 peer. osv-scanner now
reports no issues; docs site builds clean (64 pages).

Co-authored-by: bgagent <345885+scottschreckengaust@users.noreply.github.com>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

Author: scottschreckengaust