fix: recurse into NullableSchema.inner in assertValidSchema

assertValidSchema validated array items, record values, and object
property schemas but skipped the inner schema of NullableSchema. A
misconfigured nested schema like s.nullable(s.object({ x: 'bad' as any }))
would silently escape the early construction-time check and only surface
a confusing error at invocation time or, worse, produce a repair loop.

This adds a nullable branch that recurses into schema.inner before the
properties branch, making schema validation consistent for all composite
schema types.

Discovered while running sample 13-test-plan.ts: the sample uses
s.nullable indirectly through s.object with enum fields, and analysing
assertValidSchema revealed the gap.

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

Author: github-actions[bot]

skills/rig/rig.ts

@@ -1687,6 +1687,10 @@ function assertValidSchema(schema: Schema, agentName: string, slot: "input" | "o
     assertValidSchema(schema.additionalProperties, agentName, slot, `${path}.*`);
     return;
   }
+  if ("nullable" in schema && schema.nullable === true) {
+    assertValidSchema((schema as NullableSchema).inner, agentName, slot, path);
+    return;
+  }
   if ("properties" in schema) {
     for (const [key, value] of Object.entries(schema.properties) as [string, Schema][]) {
       assertValidSchema(value, agentName, slot, `${path}.${key}`);

src/rig.test.ts

@@ -141,6 +141,11 @@ describe("agent", () => {
       name: "implicit-nested",
       input: s.object({ text: "go" as any }),
     })).toThrow(/input\.text/);
+
+    expect(() => agent({
+      name: "implicit-nullable-nested",
+      output: s.nullable(s.object({ score: "bad" as any })),
+    })).toThrow(/output\.score/);
   });
 
   it("does not expose deprecated hook APIs in core", () => {