Add fx to the install instructions

[suarezesteban]

Add fx to the install instructions

fx (https://fx.sh, vercel-labs/fx) discovers skills from .agents/skills and
.claude/skills, both in the workspace and in the home directory. The toolkit
already works there with no changes on either side, so this is a docs request
rather than a feature request.

Verified with fx 0.0.8. After dropping the toolkit's skills/ into a skills
root, fx lists all 21 skills, and shopify-dev runs end to end: it searched the
docs, called shopify-admin/scripts/validate.mjs, and validated a
productCreate mutation against the 2026-07 schema, reporting the required
write_products scope.

Suggested entry for the Install section, in the same shape as the others:

• For fx: In your terminal, copy the skills into the shared agents directory:

  git clone --depth 1 https://github.com/Shopify/shopify-ai-toolkit /tmp/shopify-ai-toolkit
  mkdir -p ~/.agents/skills && cp -R /tmp/shopify-ai-toolkit/skills/* ~/.agents/skills/
  

  fx picks them up on the next start. For a single project, use .agents/skills/
  in the repository root instead.

Opening this as an issue since the README says pull requests are closed
automatically. Happy to provide the wording in whatever form is easiest.