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/shared/keys/hmac_sha256.go

@@ -0,0 +1,25 @@
+package keys
+
+import (
+	"crypto/hmac"
+	"crypto/sha256"
+	"encoding/hex"
+)
+
+type HMACSha256Hashing struct {
+	key []byte
+}
+
+func NewHMACSHA256Hashing(key []byte) *HMACSha256Hashing {
+	return &HMACSha256Hashing{key: key}
+}
+
+func (h *HMACSha256Hashing) Hash(content []byte) (string, error) {
+	mac := hmac.New(sha256.New, h.key)
+	_, err := mac.Write(content)
+	if err != nil {
+		return "", err
+	}
+
+	return hex.EncodeToString(mac.Sum(nil)), nil
+}