docs: zod missing from installation steps in openrouter-typescript-sdk and openrouter-agent-migration

Problem

Both openrouter-typescript-sdk/SKILL.md and openrouter-agent-migration/SKILL.md document tool examples that import from zod:

import { z } from 'zod'; // line 412, 1137, 1213 in typescript-sdk

But neither skill's installation section mentions npm install zod. The typescript-sdk install section only covers:

npm install @openrouter/agent
npm install @openrouter/sdk

Impact

Agents or developers copying the documented tool patterns get an immediate Cannot find module 'zod' error with no indication that an extra install step is needed.

Fix

Add npm install zod to the installation section of both skills, or add a note at the top of any code block that uses zod.

Reviewed by Perry