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/sha256.go

@@ -0,0 +1,30 @@
+package keys
+
+import (
+	"crypto/sha256"
+	"encoding/base64"
+	"fmt"
+)
+
+type Sha256Hashing struct{}
+
+func NewSHA256Hashing() *Sha256Hashing {
+	return &Sha256Hashing{}
+}
+
+func (h *Sha256Hashing) Hash(key []byte) string {
+	hashBytes := sha256.Sum256(key)
+
+	hash64 := base64.RawStdEncoding.EncodeToString(hashBytes[:])
+
+	return fmt.Sprintf(
+		"$sha256$%s",
+		hash64,
+	)
+}
+
+func (h *Sha256Hashing) HashWithoutPrefix(key []byte) string {
+	hashBytes := sha256.Sum256(key)
+
+	return base64.RawStdEncoding.EncodeToString(hashBytes[:])
+}