[rig-sampler] feat(s): add s.null schema helper for the JSON null literal type

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Sample run

Sample: src/samples/07-summarize-many-files.ts

The agent uses s.object({ files: s.string }) as input (with a p.bash(...) prompt intent) and s.object({ summary: s.string }) as output. The run completed in 1 turn with no repair loops — clean success.

What the run revealed

While the sample itself ran smoothly, reading the code during analysis exposed a gap in the s.* schema helper API: there is no s.null helper for the JSON null literal type.

null is a valid JSON value (it appears in the Json type union), yet authors had no way to declare a null-typed output field or a nullable slot in a schema. The only workaround was to use s.unknown, which is too loose and disables validation entirely.

Change

• New type: NullSchema = { type: "null"; description?: string }
• Schema union: NullSchema added alongside the other primitives
• InferSchema: T extends { type: "null" } ? null branch added for correct TypeScript inference
• validateSchema: schema.type === "null" check — accepts only null, rejects everything else with a clear error message
• s.null: helper added to the exported s object, callable as s.null or s.null("description"), consistent with all other primitive helpers
• createTypedPrimitiveSchema constraint: extended to include NullSchema
• Tests: 5 new tests covering serialization, validation acceptance/rejection, TypeScript inference, and use as an object field

Why this improves the harness

Without s.null, schema authors writing agents that reset a field, clear a value, or emit an explicit "no result" were forced to use s.unknown (disabling validation) or s.optional (which models absence, not null). s.null is a direct, self-documenting solution that slots neatly into the existing s.* API surface with no breaking changes.

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