Add a Zod schema adapter for prompt inputs

[Cryptoteep]

Context

promptkit's definePrompt currently accepts a free-form validate function. That's flexible but verbose for the common case where you just want to declare a schema and get types + validation for free.

Proposal

Add an optional schema adapter so users can write:

import { z } from 'zod'
import { definePrompt } from 'promptkit/zod'

const summarize = definePrompt({
  name: 'summarize',
  schema: z.object({
    text: z.string().min(1),
    maxLength: z.number().default(100),
  }),
  template: ({ text, maxLength }) =>
    `Summarize in ${maxLength} words: ${text}`,
})

The Zod schema would be used to:

1. Validate input (replacing/combining with validate).
2. Provide the TS type for template via z.infer.

Implementation notes

• Put it in src/adapters/zod.ts, exported via promptkit/zod (a subpath export — needs a new entry in package.json exports).
• Zod should be a peer dependency, not a runtime dep — keep the core zero-dep.
• Add tests in tests/adapters/zod.test.ts.
• Mirror the pattern for Valibot and ArkType in follow-up issues.

Acceptance

• promptkit/zod subpath export works.
• schema option infers template input type.
• Validation errors are wrapped in ValidationError.
• Tests + docs entry.

Good first issue?

Yes — well-scoped, mostly additive. Happy to pair on the design.

/label "good first issue" "help wanted" "adapter" "enhancement"