infra(matrix): CI validation + local integration test for #166/#183

- Add Gitea workflow to validate matrix scaffold on every push/PR (#183)
- Add docker-compose.test.yml for local Conduit testing (#166)
- Add test-local-integration.sh: end-to-end Hermes Matrix adapter
  proof without requiring public DNS/domain

This makes #183 self-enforcing and proves #166 is execution-ready
pending only the host/domain decision in #187.

infra/matrix/docker-compose.test.yml

@@ -0,0 +1,45 @@
+# Local integration test environment for Matrix/Conduit + Hermes
+# Issue: #166 — proves end-to-end connectivity without public DNS
+#
+# Usage:
+#   docker compose -f docker-compose.test.yml up -d
+#   ./scripts/test-local-integration.sh
+#   docker compose -f docker-compose.test.yml down -v
+
+services:
+  conduit-test:
+    image: matrixconduit/conduit:latest
+    container_name: conduit-test
+    hostname: conduit-test
+    ports:
+      - "8448:6167"
+    volumes:
+      - conduit-test-db:/var/lib/matrix-conduit
+    environment:
+      CONDUIT_SERVER_NAME: "localhost"
+      CONDUIT_PORT: "6167"
+      CONDUIT_DATABASE_BACKEND: "rocksdb"
+      CONDUIT_ALLOW_REGISTRATION: "true"
+      CONDUIT_ALLOW_FEDERATION: "false"
+      CONDUIT_MAX_REQUEST_SIZE: "20971520"
+      CONDUIT_ENABLE_OPENID: "false"
+    healthcheck:
+      test: ["CMD", "wget", "-qO-", "http://localhost:6167/_matrix/client/versions"]
+      interval: 5s
+      timeout: 3s
+      retries: 10
+
+  element-test:
+    image: vectorim/element-web:latest
+    container_name: element-test
+    ports:
+      - "8080:80"
+    environment:
+      DEFAULT_HOMESERVER_URL: "http://localhost:8448"
+      DEFAULT_HOMESERVER_NAME: "localhost"
+    depends_on:
+      conduit-test:
+        condition: service_healthy
+
+volumes:
+  conduit-test-db: