feat: add pre-commit configuration (#9)

- add pre-commit-config
- add yamllint config
- add ci/validate-* custom scripts
  - verify no secrets added
  - verify clusters with kustomize and kubeconform
  - verify apps with kustomize and kubeconform

Reviewed-on: #9

.yamllint.yaml

@@ -0,0 +1,29 @@
+# .yamllint.yaml
+extends: default
+
+rules:
+  # Allow long lines for base64 encoded values and URLs
+  line-length:
+    max: 200
+    allow-non-breakable-words: true
+    allow-non-breakable-inline-mappings: true
+
+  # Kubernetes manifests use 2-space indentation
+  indentation:
+    spaces: 2
+    indent-sequences: consistent
+
+  # Allow multiple documents (---) in a single file
+  document-start: enable
+
+  # Be lenient with comments
+  comments:
+    require-starting-space: true
+    min-spaces-from-content: 1
+
+  # Allow empty values (common in Kustomize patches)
+  empty-values: enable
+
+  truthy:
+    # Allow 'on' and 'yes' values (common in Kubernetes)
+    allowed-values: ['true', 'false', 'yes', 'no', 'on', 'off']