[rig-sampler] Add s.date schema helper for ISO 8601 date strings

[github-actions]

Sample runs (36–40)

Sample	Result	Repair turns	Notes
36-subagent-delegation.ts	✅ passed	0	Two-agent chain (researcher → planner) and a root orchestrator with agents:{} delegation. Schema fit was good.
37-output-with-nullable.ts	✅ passed	0	Git author stats with firstCommit/lastCommit as plain s.string. Date fields lacked a dedicated schema type; any string would pass validation even if the model returned a prose description instead of a date.
38-exact-literal-output.ts	✅ passed	0	Companion markdown file 138-ts-path-alias-validator.md exceeded the 30-line code limit (37 lines) — pre-existing test failure unrelated to this PR. Core sample output was correct.
39-unknown-raw-output.ts	✅ passed	0	s.enum("review-finding") used for a single-value discriminant field. s.literal would be cleaner but the sample worked fine. Companion markdown also over 30 lines — pre-existing issue.
40-record-output.ts	✅ passed	0	Two rig.agent.ask events emitted — one for the inline await extractJson(p.bash(...)) call and one for the default export invocation. Expected behaviour.

Improvement: s.date

What: adds s.date to the s.* schema helpers — a StringSchema with format: "date". The runtime validator checks that the value matches /^\d{4}-\d{2}-\d{2}$/ and returns a clear error message (expected a date string in YYYY-MM-DD format) on mismatch.

Why this run evidence supports it: sample 37 uses s.string for firstCommit and lastCommit commit-date fields. Without s.date, a model can return any string (e.g. "about 2 years ago" or "2024-03-15T00:00:00Z") and pass validation silently. With s.date, a non-conforming value fails fast on the first turn with an actionable error, reducing the chance of a silent schema mismatch reaching the caller.

Scope: additive only — no existing s.* helpers changed, no public API removed. The serialized JSON Schema already supports format: "date" (standard JSON Schema vocabulary); this just exposes a convenient typed factory and wires the validator.

Tests added (src/rig.test.ts):

• s.date serializes to {type:"string", format:"date"}
• accepts a valid YYYY-MM-DD string
• rejects a non-conforming string with a YYYY-MM-DD hint in the error
• rejects a datetime string with a time component
• usable as an object field

Generated by Daily Rig Sampler · sonnet46 100.6 AIC · ⌖ 8.47 AIC · ⊞ 5.4K · ◷