docs: add the missing cast to .input() in the Task 3 plan snippet

The builder's .input() did not typecheck as written (TS2345). The phantom
__input/__output markers make ProcedureDef invariant, which is exactly why
.output<T>() already carried a cast - .input() needed the analogous one and
did not have it.

Caught by the Task 3 implementer, who also verified via @ts-expect-error that
InferProcedureInput/InferProcedureOutput genuinely reject wrong shapes, so
the phantom markers are carrying real type information rather than silently
widening.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/plans/2026-08-05-inter-app-comms-implementation.md

@@ -561,7 +561,14 @@ export class ProcedureBuilder<Input, Output> {
   }
 
   input<S extends ObjectSchema<object>>(schema: S): ProcedureBuilder<InferInput<S>, Output> {
-    return new ProcedureBuilder<InferInput<S>, Output>({ ...this.def, input: schema });
+    // The cast is required for the same reason .output<T>() needs one: the
+    // phantom __input/__output markers make ProcedureDef invariant, so
+    // spreading a ProcedureDef<Input, Output> into a ProcedureDef<InferInput<S>,
+    // Output> is not assignable without it. No runtime effect.
+    return new ProcedureBuilder<InferInput<S>, Output>({
+      ...this.def,
+      input: schema,
+    } as ProcedureDef<InferInput<S>, Output>);
   }
 
   output<T>(): ProcedureBuilder<Input, T> {