docs: add a Security section covering supply chain and blast radius

Handing an agent credentials to a live storefront warrants a straight
answer about what it can reach and what it depends on, and that material
was previously scattered across Features bullets, a Docker paragraph and
the Verify section.

Gives the honest dependency figure rather than the flattering one: two
direct dependencies, but ~94 packages transitively, all via the official
MCP SDK. A reader running npm ls sees 94, so claiming 'two dependencies'
reads as overselling. What actually shrinks the exposure is that no
package in the tree runs an install script, and only 5 are reachable at
runtime because the server speaks stdio and never imports the SDK's
express/OAuth stack. Every claim ships with a command to check it.

Separates the two kinds of read-only guarantee, which are easy to
conflate: assertReadOnly is a property of this code and only holds while
the code is what you think it is, whereas the app's access scopes are
enforced by Shopify regardless. Read-only scopes backstop the code.