fix(builder): separate build output from error to preserve timeout messages

When a build timed out, the timeout error was silently replaced by
truncated stderr output. Split into separate Error and Output fields
on BuildHostResult so the cause (e.g. "build timed out after 30m0s")
is always visible in logs and CLI output.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/builder/builder.go

@@ -269,10 +269,12 @@ func (b *Builder) handleBuildRequest(subject string, data []byte) {
 			DurationSeconds: hostDuration,
 		}
 		if !result.Success {
-			hostResult.Error = truncateOutput(result.Stderr, 50)
-			if hostResult.Error == "" && result.Error != nil {
+			if result.Error != nil {
 				hostResult.Error = result.Error.Error()
 			}
+			if result.Stderr != "" {
+				hostResult.Output = truncateOutput(result.Stderr, 50)
+			}
 		}
 		results = append(results, hostResult)
 
@@ -284,7 +286,7 @@ func (b *Builder) handleBuildRequest(subject string, data []byte) {
 			}
 		} else {
 			failed++
-			b.logger.Error("host build failed", "host", host, "error", hostResult.Error)
+			b.logger.Error("host build failed", "host", host, "error", hostResult.Error, "output", hostResult.Output)
 			if b.metrics != nil {
 				b.metrics.RecordHostBuildFailure(req.Repo, host, hostDuration)
 			}