Add devcontainer for cross-platform development

- Dockerfile with all required tools (kubectl, flux, helm, sops, age, etc.)
- devcontainer.json with VSCode config and extensions
- postCreateCommand.sh for setup verification
- Comprehensive README with setup instructions for macOS, Windows/WSL2, Linux
- Automatic mounts for kubeconfig, SSH keys, age encryption keys
- SOPS_AGE_KEY_FILE and KUBECONFIG pre-configured

Enables development on Windows, macOS, and Linux with consistent environment.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

.devcontainer/devcontainer.json

@@ -0,0 +1,47 @@
+{
+  "name": "ESS Community GitOps",
+  "build": {
+    "dockerfile": "Dockerfile",
+    "context": "."
+  },
+  "mounts": [
+    "source=${localEnv:HOME}/.kube,target=/home/vscode/.kube,type=bind,consistency=cached",
+    "source=${localEnv:HOME}/.ssh,target=/home/vscode/.ssh,type=bind,consistency=cached",
+    "source=${localEnv:HOME}/.age,target=/home/vscode/.age,type=bind,consistency=cached",
+    "source=/var/run/docker.sock,target=/var/run/docker.sock,type=bind"
+  ],
+  "remoteUser": "vscode",
+  "features": {
+    "ghcr.io/devcontainers/features/git:1": {},
+    "ghcr.io/devcontainers/features/github-cli:1": {}
+  },
+  "remoteEnv": {
+    "KUBECONFIG": "/home/vscode/.kube/config",
+    "SOPS_AGE_KEY_FILE": "/home/vscode/.age/keys.txt"
+  },
+  "customizations": {
+    "vscode": {
+      "extensions": [
+        "ms-kubernetes-tools.vscode-kubernetes-tools",
+        "redhat.vscode-yaml",
+        "redhat.vscode-commons",
+        "monokai.theme-monokai-pro-vscode",
+        "eamodio.gitlens"
+      ],
+      "settings": {
+        "[yaml]": {
+          "editor.defaultFormatter": "redhat.vscode-yaml",
+          "editor.formatOnSave": true,
+          "editor.tabSize": 2
+        },
+        "yaml.schemas": {
+          "https://json.schemastore.org/kustomization.json": "**/kustomization.yaml",
+          "https://json.schemastore.org/helmrelease.json": "**/*helmrelease*.yaml"
+        },
+        "editor.theme": "Monokai Pro"
+      }
+    }
+  },
+  "postCreateCommand": "bash .devcontainer/postCreateCommand.sh",
+  "forwardPorts": []
+}