Add Shopify theme install adapter #5

Goal

Make installing Jetti on a Shopify theme a one-prompt flow.

Today the install CLI doesn't auto-detect Shopify themes, so the experience falls through to manual snippet paste. Shopify themes are a big share of the 'let me get a stakeholder review on the homepage' use case — worth a first-class adapter.

Scope

• Detect when the working directory is a Shopify theme: presence of config/settings_schema.json, layout/theme.liquid, templates/, etc.
• Mint the install session via POST /api/agent-installs.
• Patch layout/theme.liquid by injecting the snippet <script> tag inside the <head> (or in a new partial like snippets/jetti.liquid and {% render 'jetti' %} it from the layout).
• Update the framework matrix in README.md — flip Shopify row from 🔜 to ✅.

Out of scope

• Shopify Admin API integration (deploy via Shopify CLI is the user's job).
• Multiple-theme support — patch the active development theme only.
• Storefront app or checkout extension flows. Snippet runs on the storefront only.

Acceptance

• Running install jetti inside a shopify theme dev working tree patches the right Liquid file and the reviewer flow works on the storefront.
• README matrix reflects Shopify as supported.

Useful pointers

• scripts/jetti-install.mjs — install CLI source.
• Shopify theme structure: https://shopify.dev/docs/storefronts/themes/architecture
• theme.liquid reference: https://shopify.dev/docs/storefronts/themes/architecture/layouts