Port envd from e2b with internalized shared packages and Connect RPC

- Copy envd source from e2b-dev/infra, internalize shared dependencies
  into envd/internal/shared/ (keys, filesystem, id, smap, utils)
- Switch from gRPC to Connect RPC for all envd services
- Update module paths to git.omukk.dev/wrenn/{sandbox,sandbox/envd}
- Add proto specs (process, filesystem) with buf-based code generation
- Implement full envd: process exec, filesystem ops, port forwarding,
  cgroup management, MMDS integration, and HTTP API
- Update main module dependencies (firecracker SDK, pgx, goose, etc.)
- Remove placeholder .gitkeep files replaced by real implementations

envd/internal/api/error.go

@@ -0,0 +1,21 @@
+package api
+
+import (
+	"encoding/json"
+	"errors"
+	"net/http"
+)
+
+func jsonError(w http.ResponseWriter, code int, err error) {
+	w.Header().Set("Content-Type", "application/json; charset=utf-8")
+	w.Header().Set("X-Content-Type-Options", "nosniff")
+
+	w.WriteHeader(code)
+	encodeErr := json.NewEncoder(w).Encode(Error{
+		Code:    code,
+		Message: err.Error(),
+	})
+	if encodeErr != nil {
+		http.Error(w, errors.Join(encodeErr, err).Error(), http.StatusInternalServerError)
+	}
+}