feat(builder): log build failure output as separate lines

Log each line of build failure output as a separate structured log entry
at WARN level, making output readable and queryable in Loki/Grafana.
Add repo and rev fields to all build-related log entries. Add
truncateOutputLines helper that returns a []string for per-line logging.

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

internal/builder/builder_test.go

@@ -1,6 +1,7 @@
 package builder
 
 import (
+	"fmt"
 	"strings"
 	"testing"
 )
@@ -83,6 +84,54 @@ func makeLines(n int) []string {
 	return lines
 }
 
+func TestTruncateOutputLines(t *testing.T) {
+	t.Run("short output returns all lines", func(t *testing.T) {
+		input := "line1\nline2\nline3"
+		got := truncateOutputLines(input, 50)
+		if len(got) != 3 {
+			t.Errorf("got %d lines, want 3", len(got))
+		}
+		if got[0] != "line1" || got[1] != "line2" || got[2] != "line3" {
+			t.Errorf("unexpected lines: %v", got)
+		}
+	})
+
+	t.Run("over threshold returns head + marker + tail", func(t *testing.T) {
+		lines := makeLines(200)
+		input := strings.Join(lines, "\n")
+		got := truncateOutputLines(input, 50)
+
+		// Should be 50 head + 1 marker + 50 tail = 101
+		if len(got) != 101 {
+			t.Errorf("got %d lines, want 101", len(got))
+		}
+
+		// Check first and last lines preserved
+		if got[0] != lines[0] {
+			t.Errorf("first line = %q, want %q", got[0], lines[0])
+		}
+		if got[len(got)-1] != lines[len(lines)-1] {
+			t.Errorf("last line = %q, want %q", got[len(got)-1], lines[len(lines)-1])
+		}
+
+		// Check omitted marker
+		marker := got[50]
+		expected := fmt.Sprintf("... (%d lines omitted) ...", 100)
+		if marker != expected {
+			t.Errorf("marker = %q, want %q", marker, expected)
+		}
+	})
+
+	t.Run("exactly at threshold returns all lines", func(t *testing.T) {
+		lines := makeLines(100)
+		input := strings.Join(lines, "\n")
+		got := truncateOutputLines(input, 50)
+		if len(got) != 100 {
+			t.Errorf("got %d lines, want 100", len(got))
+		}
+	})
+}
+
 func TestTruncateOutputPreservesContent(t *testing.T) {
 	// Create input with distinct first and last lines
 	lines := make([]string, 200)