fix(web): open WebError.code to string, aligning with other seams

The closed WebErrorCode union leaked fetch-transport details (redirect,
too-large, content-type) into the seam's shared vocabulary and made web
the only seam with a closed error-code union. Drop it and let WebError
carry an open code: string like LlmError/SubagentError; document the
codes grouped by owner (seam-neutral vs dsh-web-fetch-local transport).

Addresses tianyicui's leaky-abstraction review comment on WebErrorCode.

packages/web/web/src/index.ts

@@ -36,7 +36,6 @@ export {
 } from './types.ts'
 export type {
   WebCapabilityStatus,
-  WebErrorCode,
   WebExecContext,
   WebFetchBody,
   WebFetchProvider,