refactor(ci-cd): Replace Terraform pipeline with GitOps validation and ArgoCD sync

Delete old terraform-apply.yml (terraform fmt/init/validate/plan/apply).

Create new GitOps CI/CD:
- validate-k8s.yaml: YAML lint, kubeval, kustomize build, ArgoCD validation
- argocd-sync.yaml: Auto-sync homelab-root on main branch
- security-scan.yaml: Trivy, Polaris, secret detection
- .yamllint.yaml: YAML linting configuration

Add documentation (.forgejo/CI-CD.md) and architecture guides.

Git is now single source of truth. CI validates, ArgoCD deploys.

.yamllint.yaml

@@ -0,0 +1,33 @@
+---
+extends: default
+
+rules:
+  line-length:
+    max: 120
+    level: warning
+  indentation:
+    spaces: 2
+  brackets:
+    min-spaces-inside: 0
+    max-spaces-inside: 0
+  braces:
+    min-spaces-inside: 0
+    max-spaces-inside: 0
+  comments:
+    min-spaces-from-content: 2
+  comments-indentation: {}
+  document-end: disable
+  document-start: disable
+  empty-lines:
+    max: 3
+  empty-values:
+    forbid-in-block-mappings: true
+    forbid-in-flow-mappings: true
+  key-duplicates: enable
+  key-ordering: disable
+  new-line-at-end-of-file: enable
+  new-lines:
+    type: unix
+  trailing-spaces: enable
+  truthy:
+    level: warning