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_test.go

@@ -0,0 +1,115 @@
+package builder
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestTruncateOutput(t *testing.T) {
+	tests := []struct {
+		name      string
+		input     string
+		keepLines int
+		wantLines int
+		wantOmit  bool
+	}{
+		{
+			name:      "short output unchanged",
+			input:     "line1\nline2\nline3",
+			keepLines: 50,
+			wantLines: 3,
+			wantOmit:  false,
+		},
+		{
+			name:      "exactly at threshold unchanged",
+			input:     strings.Join(makeLines(100), "\n"),
+			keepLines: 50,
+			wantLines: 100,
+			wantOmit:  false,
+		},
+		{
+			name:      "over threshold truncated",
+			input:     strings.Join(makeLines(150), "\n"),
+			keepLines: 50,
+			wantLines: 103, // 50 + 1 (empty) + 1 (omitted msg) + 1 (empty) + 50
+			wantOmit:  true,
+		},
+		{
+			name:      "large output truncated",
+			input:     strings.Join(makeLines(1000), "\n"),
+			keepLines: 50,
+			wantLines: 103,
+			wantOmit:  true,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := truncateOutput(tt.input, tt.keepLines)
+			gotLines := strings.Split(got, "\n")
+
+			if len(gotLines) != tt.wantLines {
+				t.Errorf("got %d lines, want %d", len(gotLines), tt.wantLines)
+			}
+
+			hasOmit := strings.Contains(got, "lines omitted")
+			if hasOmit != tt.wantOmit {
+				t.Errorf("got omit marker = %v, want %v", hasOmit, tt.wantOmit)
+			}
+
+			if tt.wantOmit {
+				// Verify first and last lines are preserved
+				inputLines := strings.Split(tt.input, "\n")
+				firstLine := inputLines[0]
+				lastLine := inputLines[len(inputLines)-1]
+				if !strings.HasPrefix(got, firstLine+"\n") {
+					t.Errorf("first line not preserved, got prefix %q, want %q",
+						gotLines[0], firstLine)
+				}
+				if !strings.HasSuffix(got, lastLine) {
+					t.Errorf("last line not preserved, got suffix %q, want %q",
+						gotLines[len(gotLines)-1], lastLine)
+				}
+			}
+		})
+	}
+}
+
+func makeLines(n int) []string {
+	lines := make([]string, n)
+	for i := range lines {
+		lines[i] = "line " + strings.Repeat("x", i%80)
+	}
+	return lines
+}
+
+func TestTruncateOutputPreservesContent(t *testing.T) {
+	// Create input with distinct first and last lines
+	lines := make([]string, 200)
+	for i := range lines {
+		lines[i] = "middle"
+	}
+	lines[0] = "FIRST"
+	lines[49] = "LAST_OF_HEAD"
+	lines[150] = "FIRST_OF_TAIL"
+	lines[199] = "LAST"
+
+	input := strings.Join(lines, "\n")
+	got := truncateOutput(input, 50)
+
+	if !strings.Contains(got, "FIRST") {
+		t.Error("missing FIRST")
+	}
+	if !strings.Contains(got, "LAST_OF_HEAD") {
+		t.Error("missing LAST_OF_HEAD")
+	}
+	if !strings.Contains(got, "FIRST_OF_TAIL") {
+		t.Error("missing FIRST_OF_TAIL")
+	}
+	if !strings.Contains(got, "LAST") {
+		t.Error("missing LAST")
+	}
+	if !strings.Contains(got, "(100 lines omitted)") {
+		t.Errorf("wrong omitted count, got: %s", got)
+	}
+}