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/postCreateCommand.sh

@@ -0,0 +1,35 @@
+#!/bin/bash
+set -e
+
+echo "🚀 Setting up ESS Community GitOps devcontainer..."
+
+# Verify all required tools are installed
+echo "✅ Verifying installed tools..."
+commands=("kubectl" "flux" "helm" "sops" "age" "git" "docker")
+
+for cmd in "${commands[@]}"; do
+  if command -v $cmd &> /dev/null; then
+    version=$($cmd version 2>/dev/null | head -1 || echo "installed")
+    echo "  ✓ $cmd: $version"
+  else
+    echo "  ✗ $cmd: NOT FOUND"
+    exit 1
+  fi
+done
+
+# Create necessary directories
+echo "📁 Creating home directories..."
+mkdir -p ~/.kube ~/.ssh ~/.age
+
+# Print useful information
+echo ""
+echo "📚 Useful commands:"
+echo "  - kubectl get pods -n matrix          (check pod status)"
+echo "  - flux get helmreleases -A            (check helm releases)"
+echo "  - sops apps/production/custom-configs/mas-secrets.sops.yaml  (edit secrets)"
+echo ""
+echo "🔗 For kubeconfig setup:"
+echo "  - Copy your ~/.kube/config to access the cluster"
+echo "  - Run: kubectl get nodes"
+echo ""
+echo "✨ Devcontainer setup complete!"