refactor: modernize stdlib usage and move matchers into dsl

- Move Matcher types and matching logic from runner/matcher.go into
  the dsl package as methods on the Matcher types. Runner now calls
  t.Stdout.Match(label, actual) instead of type-switching via a
  package-level applyMatcher helper.
- Replace custom contains/containsString helpers with slices.Contains
  in dsl/build.go and runner/compiler.go.
- Use maps.Copy instead of manual map copy in BuildConfig.MergeFrom.
- Adopt strings.SplitSeq, strings.CutPrefix and the `for range N` loop
  form in runner/limiter_linux.go.
- Ignore example/imdb build artifact.

runner/limiter_linux.go

@@ -102,9 +102,9 @@ func createScopeCgroup() (string, error) {
 		return "", fmt.Errorf("read /proc/self/cgroup: %w", err)
 	}
 	var rel string
-	for _, line := range strings.Split(strings.TrimSpace(string(data)), "\n") {
-		if strings.HasPrefix(line, "0::") {
-			rel = strings.TrimPrefix(line, "0::")
+	for line := range strings.SplitSeq(strings.TrimSpace(string(data)), "\n") {
+		if after, ok := strings.CutPrefix(line, "0::"); ok {
+			rel = after
 			break
 		}
 	}
@@ -200,7 +200,7 @@ func (l *linuxLimiter) collect() limitStats {
 		}
 	}
 	if data, err := os.ReadFile(filepath.Join(l.cgPath, "memory.events")); err == nil {
-		for _, line := range strings.Split(string(data), "\n") {
+		for line := range strings.SplitSeq(string(data), "\n") {
 			fields := strings.Fields(line)
 			if len(fields) != 2 {
 				continue
@@ -217,7 +217,7 @@ func (l *linuxLimiter) cleanup() {
 	if l.cgPath == "" {
 		return
 	}
-	for i := 0; i < 10; i++ {
+	for range 10 {
 		err := os.Remove(l.cgPath)
 		if err == nil || os.IsNotExist(err) {
 			l.cgPath = ""