stripMarkdownImageAltText removed alt text from inline images
() but not reference-style images (![alt][ref]), because the
regex requires the "](" of the inline form. Alt text is a
hidden-instruction channel that reaches the prompt via sanitizeContent,
so the reference-style form let it survive.
Add a matching replace for the reference-style form (![alt][ref] ->
![][ref]), preserving the [ref] label so the image definition still
resolves. Adds regression tests.
Co-authored-by: Contributor <you@example.com>
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>