Adds --output-schema/-o for structured output across all providers (except OpenAI in background mode), using ArkType as the user-facing schema language.

The secret is https://arktype.io/, which lets you express schemas in a very concise form, which can then be converted programmatically to the JSON Schema format the SDKs want. We don't have to eval anything, we just pare with JSON5.

Provider-specific quirks: object schemas are closed, OpenAI strict mode gets all fields marked required, and non-object roots are wrapped as { value: ... } then unwrapped before printing.

Examples

ai -o "boolean" "is 17 prime?"
# true

ai -o "'yes' | 'no'" "is Paris in France?"
# yes

ai -o '{ urgent: "boolean", reason: "string" }' 'is this urgent? server is down'
# {
#   "urgent": true,
#   "reason": "The service is unavailable."
# }

ai -o "string[]" "give three terse git commit message verbs"
# ["fix","add","remove"]