fix(builder): truncate large error output to prevent log overflow

Build errors from nix can be very large (100k+ chars). This truncates
error output to the first 50 and last 50 lines when it exceeds 100
lines, preventing journal and NATS message overflow.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/builder/builder.go

@@ -6,6 +6,7 @@ import (
 	"log/slog"
 	"regexp"
 	"sort"
+	"strings"
 	"sync"
 	"time"
 
@@ -18,6 +19,18 @@ import (
 // Allows: alphanumeric, dashes, underscores, dots.
 var hostnameRegex = regexp.MustCompile(`^[a-zA-Z0-9._-]+$`)
 
+// truncateOutput truncates output to the first and last N lines if it exceeds 2*N lines.
+func truncateOutput(output string, keepLines int) string {
+	lines := strings.Split(output, "\n")
+	if len(lines) <= keepLines*2 {
+		return output
+	}
+	head := lines[:keepLines]
+	tail := lines[len(lines)-keepLines:]
+	omitted := len(lines) - keepLines*2
+	return strings.Join(head, "\n") + fmt.Sprintf("\n\n... (%d lines omitted) ...\n\n", omitted) + strings.Join(tail, "\n")
+}
+
 // BuilderConfig holds the configuration for the builder.
 type BuilderConfig struct {
 	NATSUrl        string
@@ -256,7 +269,7 @@ func (b *Builder) handleBuildRequest(subject string, data []byte) {
 			DurationSeconds: hostDuration,
 		}
 		if !result.Success {
-			hostResult.Error = result.Stderr
+			hostResult.Error = truncateOutput(result.Stderr, 50)
 			if hostResult.Error == "" && result.Error != nil {
 				hostResult.Error = result.Error.Error()
 			}