When the opencode process crashes (e.g. stale session ID causes
'Session not found'), readLoop sends a fallback EventResult before
the deferred cmd.Wait() can detect and emit an EventError. The
engine receives EventResult(Done:true) first, returns from the
event loop, and the subsequent EventError is never consumed.
The user sees an empty response with no error information.
Fix by checking stderrBuf immediately after the scanner loop exits,
before sending the fallback EventResult. If stderr contains errors
(including 'Session not found'), emit EventError instead.
cmd.Wait() remains in a defer for resource cleanup only.
Engine uses a pattern table (agentErrorHandlers) to match known
errors and show localized user-friendly messages with recovery hints.
Co-authored-by: vibecoder <16832299+vibecoder@user.noreply.gitee.com>
