Add fx to the install instructions

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.

Open the README and locate the Install section, then compare the existing entries to the suggested fx wording. Add an fx entry covering the shared and project-local skills directories, and verify that the instructions match the tested git, mkdir, and cp commands.