Add ImageMagick conversion support

[abmmhasan]

Scope

Add ImageMagick to the Tools image so LocalDevStack can expose image conversion under the unified conversion namespace:

lds convert docs  <input> <output> ...
lds convert image <input> <output> ...

Runtime

Adds:

• imagemagick;
• imagemagick-webp;
• libwebp-tools.

The WebP packages are required because Alpine's ImageMagick delegates WebP encoding through the external cwebp tool.

Validation

Final head: 4b13b5fa5dd8c451f4ffbbec94abb2ace08b57df

PR run #400: green.

The release gate verifies:

• magick exists;
• PNG creation;
• PNG -> JPEG;
• PNG -> GIF;
• PNG -> WebP;
• animated GIF creation;
• animated GIF -> animated WebP preserves multiple frames;
• animated GIF -> JPEG first-frame extraction;
• ImageMagick identify succeeds on JPEG/GIF/WebP;
• JPEG, PNG, GIF and WebP appear in magick -list format;
• amd64 and arm64 image smoke remain green;
• the full existing Tools release gate remains green.

README now documents Pandoc and ImageMagick as conversion/runtime utilities.

This PR is the runtime dependency for LocalDevStack PR #24.

No merge performed.