opencode's `ToolStateError` carries the failure reason on `state.error`,
not `state.output`. our log handler was reading `state.output` and
falling back to `(no error message)`, so every tool failure logged a
useless line. type the state as a discriminated union (mirrors
@opencode-ai/sdk) so the field misread becomes a compile error.
operator-facing only: the model already received the real error via
opencode's tool-result envelope (verified by running webfetch against
a known-404 URL — model reported "Error: Request failed with status
code: 404" verbatim).
closes#662
Colin McDonnell
8d6460da1c