fix: rewrite Tekton integration tests for X-Service routing

FIXES:
- Remove stale files: k8s/argocd-apps/, k8s/tekton/base/, overlays/
  (Tekton infra is in homelab repo, not here)
- Fix step.resources → step.computeResources (Tekton v1 API)
- Fix Task: use curl sidecar pattern instead of distroless image
  (distroless has no shell/curl/go)
- Fix routing: use X-Service + X-Resource headers, not path-based
- Extract test script to scripts/integration-test.sh (ConfigMap mount)
- Install kubectl in CI runner (was missing)
- Prune README to essentials

TASK ARCHITECTURE:
  sidecar: gateway image (mounts config secret, runs on localhost)
  step: curlimages/curl (runs integration-test.sh from ConfigMap)

TEST COVERAGE:
  health, header validation, memory, s3, sqs, workflow, iam

k8s/tekton/kustomization.yaml

@@ -1,7 +1,5 @@
 apiVersion: kustomize.config.k8s.io/v1beta1
 kind: Kustomization
-metadata:
-  name: api-gateway-tekton
 
 namespace: api
 
@@ -9,7 +7,10 @@ resources:
 - task-integration-test.yaml
 - pipeline-integration-test.yaml
 
-commonLabels:
-  app: api-gateway
-  component: testing
-  managed-by: argocd
+generatorOptions:
+  disableNameSuffixHash: true
+
+configMapGenerator:
+- name: integration-test-script
+  files:
+  - scripts/integration-test.sh