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.