refactor: swap unified pipeline for Satteri

[erinxocon]

Summary

Moves markdown processing off unified/remark/rehype and onto Satteri's native mdast pipeline. Also drops the third-party astro-rehype-relative-markdown-links in favor of our own @system76/satteri-relative-markdown-links@1.0.0, and picks up a minor Astro bump along the way.

Changes

• astro.config.mjs: unified({ remarkPlugins, rehypePlugins }) → satteri({ mdastPlugins }).
• Deps: drop @astrojs/markdown-remark, astro-rehype-relative-markdown-links, and direct unified; add @astrojs/markdown-satteri ^0.3.3, satteri ^0.9.4, and @system76/satteri-relative-markdown-links ^1.0.0. Bump astro ^7.0.3 → ^7.0.6 and pnpm 11.9.0 → 11.10.0.
• Rename src/plugins/remark-wrap-images-with-originals.ts → src/plugins/satteri-wrap-images-with-originals.ts and rewrite as a Satteri mdast plugin: defineMdastPlugin with an image visitor and ctx.wrapNode, replacing the hand-rolled recursive walk. Source of the containing markdown file now comes from ctx.fileURL (via fileURLToPath) rather than file.history[0].
• pnpm-workspace.yaml: add a minimumReleaseAgeExclude entry for @system76/satteri-relative-markdown-links@1.0.0 so the cooldown gate doesn't hold back the fresh internal publish.