webfetch collapses all errors to one message and hard-fails on >5MB responses

Describe the bug

Two related error-handling gaps in webfetch:

1. Every failure collapses to one message. Timeout, >5MB response, unsupported content-type, and HTTP 403/404 all reach the model as the identical Unable to fetch <url> (the Effect.mapError at the end of execute discards the error channel, unlike skill.ts which passes the error through). The model can't distinguish retryable from non-retryable and can't adapt.

2. Oversized responses hard-fail instead of truncating. MAX_RESPONSE_BYTES (5MB) is enforced via Effect.fail in collectBoundedResponseBody, so a page over 5MB (common with inlined JSON SPA data) returns nothing at all — despite the tool description promising "Large text results may be replaced with a preview".

Affected code

packages/core/src/tool/webfetch.ts — final Effect.mapError; collectBody/http-body.ts size limit.

Suggested fix

Preserve the specific error message, and truncate/preview oversized responses instead of hard-failing.

Environment

• opencode version: latest dev