test: tests para soporte de threads de Matrix

Tests unitarios:
- runner_test.go: verifica ruteo correcto de ReplyAction segun
  ThreadID (plain markdown, reply, thread, thread sin fallback, nil reply)
- thread_test.go: extraccion de ThreadID desde m.relates_to raw
  (thread, reply sin thread, plain, m.replace, thread sin event_id)
- thread_relates_test.go: estructura JSON de RelatesTo.SetThread
  cumple la spec de Matrix (rel_type, event_id, is_falling_back, m.in_reply_to)

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

shell/matrix/thread_test.go

@@ -0,0 +1,95 @@
+package matrix
+
+import (
+	"testing"
+)
+
+// extractThreadID is the pure logic extracted from the listener for testability.
+// It mirrors the extraction in listener.go's EventMessage handler.
+func extractThreadID(rawContent map[string]any) string {
+	relatesTo, ok := rawContent["m.relates_to"].(map[string]any)
+	if !ok {
+		return ""
+	}
+	relType, _ := relatesTo["rel_type"].(string)
+	if relType != "m.thread" {
+		return ""
+	}
+	threadRoot, _ := relatesTo["event_id"].(string)
+	return threadRoot
+}
+
+func TestExtractThreadID_ThreadMessage(t *testing.T) {
+	raw := map[string]any{
+		"body": "hello",
+		"m.relates_to": map[string]any{
+			"rel_type": "m.thread",
+			"event_id": "$root123",
+			"is_falling_back": true,
+			"m.in_reply_to": map[string]any{
+				"event_id": "$last456",
+			},
+		},
+	}
+
+	got := extractThreadID(raw)
+	if got != "$root123" {
+		t.Errorf("expected $root123, got %q", got)
+	}
+}
+
+func TestExtractThreadID_ReplyNotThread(t *testing.T) {
+	raw := map[string]any{
+		"body": "hello",
+		"m.relates_to": map[string]any{
+			"m.in_reply_to": map[string]any{
+				"event_id": "$evt789",
+			},
+		},
+	}
+
+	got := extractThreadID(raw)
+	if got != "" {
+		t.Errorf("expected empty string for non-thread reply, got %q", got)
+	}
+}
+
+func TestExtractThreadID_NoRelatesTo(t *testing.T) {
+	raw := map[string]any{
+		"body": "plain message",
+	}
+
+	got := extractThreadID(raw)
+	if got != "" {
+		t.Errorf("expected empty string for plain message, got %q", got)
+	}
+}
+
+func TestExtractThreadID_ReplaceRelation(t *testing.T) {
+	raw := map[string]any{
+		"body": "edited",
+		"m.relates_to": map[string]any{
+			"rel_type": "m.replace",
+			"event_id": "$original",
+		},
+	}
+
+	got := extractThreadID(raw)
+	if got != "" {
+		t.Errorf("expected empty string for m.replace, got %q", got)
+	}
+}
+
+func TestExtractThreadID_ThreadWithoutEventID(t *testing.T) {
+	raw := map[string]any{
+		"m.relates_to": map[string]any{
+			"rel_type": "m.thread",
+			// missing event_id
+		},
+	}
+
+	got := extractThreadID(raw)
+	if got != "" {
+		t.Errorf("expected empty string for thread without event_id, got %q", got)
+	}
+}