fix(mdx): declare satteri as a direct dependency

[grant]

Changes

• Moves satteri from @astrojs/mdx devDependencies to dependencies so the bare import … from 'satteri' in the Sätteri MDX entrypoints is declared for consumers
• Adds a changeset

@astrojs/mdx statically imports satteri from src/satteri/*, but previously relied on it only transitively via the optional @astrojs/markdown-satteri peer (or a devDependency that is not published to consumers). Under pnpm’s isolated node_modules, that can fail with ERR_MODULE_NOT_FOUND for the bare satteri import during astro build of MDX pages.

Fixes #17371

Testing

• Minimal Astro + MDX repro: https://github.com/grant/astro-mdx-satteri-undeclared-dep-repro
  (pnpm install && pnpm repro runs a real astro build, then again after removing .pnpm/node_modules/satteri)
• Lockfile updated with pnpm install --lockfile-only

[grant]

@astrobot-houston