Initial project structure for Wrenn Sandbox

Set up directory layout, Makefiles, go.mod files, docker-compose,
and empty placeholder files for all packages.

.env.example

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+# Database
+DATABASE_URL=postgres://wrenn:wrenn@localhost:5432/wrenn?sslmode=disable
+
+# Control Plane
+CP_LISTEN_ADDR=:8000
+CP_HOST_AGENT_ADDR=localhost:50051
+
+# Host Agent
+AGENT_LISTEN_ADDR=:50051
+AGENT_KERNEL_PATH=/var/lib/wrenn/kernels/vmlinux
+AGENT_IMAGES_PATH=/var/lib/wrenn/images
+AGENT_SANDBOXES_PATH=/var/lib/wrenn/sandboxes
+AGENT_HOST_INTERFACE=eth0
+
+# Lago (billing — external service)
+LAGO_API_URL=http://localhost:3000
+LAGO_API_KEY=
+
+# Object Storage (hibernate snapshots — Hetzner Object Storage, S3-compatible)
+S3_BUCKET=wrenn-snapshots
+S3_REGION=fsn1
+S3_ENDPOINT=https://fsn1.your-objectstorage.com
+AWS_ACCESS_KEY_ID=
+AWS_SECRET_ACCESS_KEY=