fix: name run_code's required description argument in its model-facing prose

The transport schema requires both `code` and `description`, but the tool
description and both SDK instruction flavors described the call as passing a
program. `description` was reachable only through the parameter schema, so a
model following the prose emitted `{code}` alone and lost the whole written
program to an INVALID_ARGS rejection.

The length and format guidance stays in RUN_CODE_DESCRIPTION_PARAM_DESCRIPTION
alone, so the schema and the prompt cannot drift.

Fixes #2426

.agents/notes/implemented/feature/2026-06-15-code-mode.md

@@ -87,6 +87,8 @@ The worker runtime provides containment, not a security boundary: model code can
 
 The SDK instructs the model to write an async body in the loaded runtime's language (an erasable-TypeScript body by default; a Python `async` body under a Python runtime — see the [language-dispatch note](2026-07-31-code-mode-language-dispatch.md)), call tools through `await tools.name(args)`, catch rejected tool calls when needed, and return or log only the output that should re-enter context. Both flavors state the same contract in their own primitive: independent read-only calls MAY overlap under `Promise.all` (TypeScript) or `asyncio.gather` (Python), mutating calls run alone in submission order, and dependent work sequences with `await`. The declaration prefix can be as large as native schemas, especially in `'both'`, but remains stable for provider caching.
 
+The transport's own `description` and both SDK instruction flavors open by naming `code` and `description` as the call's two required arguments. Prose that describes the call as passing a program leaves the second argument discoverable only through the parameter schema, and a model that emits `{code}` alone loses the whole written program to an `INVALID_ARGS` rejection.
+
 ## Consequences
 
 Deployments switching to `'code'` must update any native-only `toolOrder`. Assembly listeners own the integrity of any rewritten protocol messages. Sub-dispatch starts in submission order under a bounded overlap pool, while per-call contexts retain their source, envelope, and metadata through the outer result.