Update README to reflect actual CLI behavior and Shop skill companionship

[joerj123]

What

Rewrites the README to accurately describe how the shop CLI actually works, based on the current command surface in src/cli.ts, and adds a nod to the fact that the CLI is the companion to the Shop skill (https://shop.app/SKILL.md).

Why

The README had drifted from the implementation — it was missing several commands and flags, and did not mention its relationship to the Shop skill that drives the end-to-end shopping flow.

Changes

• Companion to the Shop skill — new section linking to https://shop.app/SKILL.md and explaining the skill drives the shopping conversation while the CLI does the work underneath.
• Global options — documents --country (context signal, not a ships-to filter), --profile-url, --format md|json, --memory-store, plus the real output-format behavior (auth/checkout → JSON, orders → markdown, catalog → md/json).
• Search — full, accurate flag set (minor-unit price bounds, taxonomy color/size/gender filters, ships-from/to/region/postal, --like-id, --image, etc.).
• Auth — adds the previously undocumented non-blocking device-code → poll flow alongside blocking login; notes tokens live in the OS secret store via keytar.
• Checkout — adds the missing checkout update command, create-from-cart path, the --confirm refusal behavior, messages[] warning surfacing, and --buyer-ip / SHOP_BUYER_IP.
• Orders — full --type set including order_info, plus --date-from/--date-to/--cursor.
• Config — adds the undocumented config set-country and config show commands.
• Intro/Install — describes the APIs it talks to, Node >= 20 requirement, and from-source install.

Docs-only change.