Add CategorySchema validator

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Goal

Add a contract validator for category schema definitions.

Memact needs category schemas to be shaped consistently so Schema, SDK, Access, Wiki, and Memory can rely on the same structure.

Suggested fields

A category schema may include:

• schema id
• category name
• version
• description
• context fields
• example raw inputs
• normalized output shape
• Wiki entry templates
• permission suggestions
• safety notes

Work to do

• Define the CategorySchema shape
• Add a validator following the existing Contracts style
• Return { ok: true, value } for valid input
• Return { ok: false, errors: [{ path, message }] } for invalid input
• Add valid examples
• Add invalid examples
• Export the validator from the package

Acceptance criteria

• Valid category schemas pass
• Missing required fields fail with useful errors
• Invalid field types fail with useful errors
• The validator is documented in the README or examples

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