feat: add s.nonEmptyObject schema helper and p.writeOutput prompt helper

[Copilot]

From the daily rig evaluation, two gaps were identified: no way to reject empty {} map outputs at validation time, and no clean way to wire an LLM-generated output field to a file path (the existing p.write takes a static string, creating ambiguity about who provides the content).

s.nonEmptyObject(valueSchema, description?)

Constrained RecordSchema with minProperties: 1 — mirrors s.nonEmptyArray for maps.

output: s.object({
  labels: s.nonEmptyObject(s.string, "at least one label required"),
})
// serializes to { type: "object", additionalProperties: { type: "string" }, minProperties: 1 }
// validation rejects {}

• RecordSchema type gains an optional minProperties field
• serializeSchema emits minProperties in JSON Schema output
• validateSchema enforces the constraint at runtime

p.writeOutput(field, path, options?)

New prompt intent (mode: "prompt.writeOutput") that tells the harness to write the value of a named output field to a file after the agent generates its response — solving the p.write content-source ambiguity.

const tracker = agent({
  instructions: p`Scan for TODO comments. ${p.writeOutput("report", "todo-report.md")}`,
  output: s.object({ report: s.string }),
});
// renders: "After generating the response, write the value of output field "report" to the file at path "todo-report.md""

Use p.writeOutput when the content is LLM-generated; use p.write for static content.

Docs

• SKILL.md and README.md updated with both helpers, usage examples, and a p.write vs p.writeOutput guidance note
• rig.ts file-header comment updated

• Fixes [rig-tasks] Daily rig evaluation — 2026-07-23 — 5/5 passed #72

[pelikhan]

@copilot resolve the merge conflicts on this branch.

[Copilot]

@copilot resolve the merge conflicts on this branch.

Resolved. The two conflicts were both in the rig.ts file header comment:

• T:PromptIntent kind list: merged writeOutput (this PR) + env (main)
• p.write doc line: kept main's clarifying note and added p.writeOutput entry from this PR

All 189 tests pass (bd37a90).