Add s.date schema helper for ISO 8601 date strings

Motivated by sample-run analysis (samples 36–40): sample 37
(git-author-stats) uses plain s.string for firstCommit/lastCommit date
fields. A dedicated s.date helper (format:"date", validated as
YYYY-MM-DD) makes the intent explicit and catches non-conforming
strings before they reach repair turns.

Changes:
- Add s.date to the s.* helpers: string schema with format:"date"
- Validate format:"date" in validateSchema with a clear YYYY-MM-DD
  error message
- Update @file header comment to document the new helper
- Add 5 unit tests in src/rig.test.ts covering serialization,
  acceptance, and rejection cases

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: github-actions[bot]

skills/rig/rig.ts

@@ -29,7 +29,7 @@
  * T:LauncherIo type {stdin,stdout,stderr} override for launcher subprocess
  * T:JsonSchemaObject type {[key:string]:unknown} plain JSON Schema object
  * s.string/number/integer/boolean/null SchemaHelperFactory primitives; call as value or fn(desc)
- * s.int alias for s.integer; s.nonEmptyString string with minLength:1; s.url string with format:"uri"; s.path string with format:"path"
+ * s.int alias for s.integer; s.nonEmptyString string with minLength:1; s.url string with format:"uri"; s.path string with format:"path"; s.date string with format:"date" validated as YYYY-MM-DD
  * s.positiveInt integer with minimum:1; s.nonNegativeInt integer with minimum:0; NumberSchema/IntegerSchema support minimum/maximum constraints
  * s.array(items,desc?) ArraySchema; use for homogeneous lists, e.g. s.array(s.string)
  * s.nonEmptyArray(items,desc?) ArraySchema with minItems:1; validates array has at least one element
@@ -241,6 +241,8 @@ export const s = {
   url: createConstrainedStringSchema({ format: "uri" }),
   /** Schema for a file system path string (format: "path"). Use instead of `s.string` when the value is a file or directory path; improves readability and hints to the runtime about path-based context resolution. Call as `s.path` or `s.path("description")`. */
   path: createConstrainedStringSchema({ format: "path" }),
+  /** Schema for an ISO 8601 calendar date string (format: "date", pattern `YYYY-MM-DD`). Use when the value is a date-only value (no time component). Validated at runtime: non-conforming strings fail with a clear error. Call as `s.date` or `s.date("description")`. */
+  date: createConstrainedStringSchema({ format: "date" }),
   /** Schema for a `number` value. Call as `s.number` or `s.number("description")`. */
   number: createTypedPrimitiveSchema<NumberSchema>("number"),
   /** Schema for an integer value. Serializes to `{"type":"integer"}` in JSON Schema. Call as `s.integer` or `s.integer("description")`. */
@@ -2059,6 +2061,11 @@ function validateSchema(value: unknown, schema: Schema, path: string, optional:
       if (format === "uri") {
         try { new URL(value); } catch { return { ok: false, error: `${path}: expected a valid URL, got ${JSON.stringify(value)}` }; }
       }
+      if (format === "date") {
+        if (!/^\d{4}-\d{2}-\d{2}$/.test(value)) {
+          return { ok: false, error: `${path}: expected a date string in YYYY-MM-DD format, got ${JSON.stringify(value)}` };
+        }
+      }
       return ok();
     }
     if (schema.type === "number") {

src/rig.test.ts

@@ -1466,6 +1466,40 @@ describe("s.path", () => {
   });
 });
 
+describe("s.date", () => {
+  it("serializes to {type:'string', format:'date'}", () => {
+    expect(toJsonSchema(s.date)).toEqual({ type: "string", format: "date" });
+    expect(toJsonSchema(s.date("release date"))).toEqual({ type: "string", format: "date", description: "release date" });
+  });
+
+  it("accepts a valid YYYY-MM-DD date string", () => {
+    const result = analyzeResponse(JSON.stringify("2024-03-15"), s.date, "test", 1);
+    expect(result.ok).toBe(true);
+  });
+
+  it("rejects a string that is not in YYYY-MM-DD format", () => {
+    const result = analyzeResponse(JSON.stringify("March 15 2024"), s.date, "test", 1);
+    expect(result.ok).toBe(false);
+    if (!result.ok) {
+      expect(result.error.message).toContain("YYYY-MM-DD");
+    }
+  });
+
+  it("rejects a datetime string with time component", () => {
+    const result = analyzeResponse(JSON.stringify("2024-03-15T10:00:00Z"), s.date, "test", 1);
+    expect(result.ok).toBe(false);
+  });
+
+  it("is usable as an object field", () => {
+    const schema = s.object({ createdAt: s.date });
+    expect(toJsonSchema(schema)).toEqual({
+      type: "object",
+      properties: { createdAt: { type: "string", format: "date" } },
+      required: ["createdAt"],
+    });
+  });
+});
+
 describe("s.positiveInt", () => {
   it("serializes to {type:'integer', minimum:1}", () => {
     expect(toJsonSchema(s.positiveInt)).toEqual({ type: "integer", minimum: 1 });