Add Shopify channel

[wby1121]

Add Shopify Channel

This PR introduces a new shopify channel to Agent-Reach.

Features

• health check
• products list
• products get

Design

• follows existing channel patterns
• uses Shopify REST API
• supports mock mode when credentials are not provided

Testing

• full test coverage in test_shopify.py
• all tests pass offline (no external dependencies)

Notes

• no changes to existing channels
• no external CLI dependencies
• minimal and focused implementation

[wby1121]

A couple of clarifications that may help review this addition:

• This channel is mock-first by default. If SHOPIFY_SHOP / SHOPIFY_TOKEN are not provided, it does not require any setup and returns deterministic mock responses for health, products list, and products get.
• The credentialed Shopify API path is an optional enhancement mode, not the default usage path.
• The intent is not web scraping or price comparison. Shopify is treated as a structured business data source, allowing agents to access store data in a predictable JSON shape.
• This complements public information channels like YouTube and Reddit by covering private, agent-usable operational data.
• Scope is intentionally narrow: read-only commands, no install flow changes, no external CLI dependency, and offline tests only.