CHORE: Remove Tekton CI/CD platform

- Delete tekton/ directory (pipeline, triggers, dashboard, ci-cd)
- Remove tekton references from kustomization.yaml
- Switching to GitHub Actions for CI/CD

tekton/ci-cd/manifests/triggers/secret.yaml

@@ -1,18 +0,0 @@
-apiVersion: external-secrets.io/v1
-kind: ExternalSecret
-metadata:
-  name: github-webhook-secret
-  namespace: tekton-pipelines
-spec:
-  refreshInterval: 1h
-  secretStoreRef:
-    name: vault-backend
-    kind: ClusterSecretStore
-  target:
-    name: github-webhook-secret
-    creationPolicy: Owner
-  data:
-    - secretKey: webhook-secret
-      remoteRef:
-        key: tekton
-        property: GITHUB_WEBHOOK_SECRET