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/sha512.go

@@ -0,0 +1,20 @@
+package keys
+
+import (
+	"crypto/sha512"
+	"encoding/hex"
+)
+
+// HashAccessToken computes the SHA-512 hash of an access token.
+func HashAccessToken(token string) string {
+	h := sha512.Sum512([]byte(token))
+
+	return hex.EncodeToString(h[:])
+}
+
+// HashAccessTokenBytes computes the SHA-512 hash of an access token from bytes.
+func HashAccessTokenBytes(token []byte) string {
+	h := sha512.Sum512(token)
+
+	return hex.EncodeToString(h[:])
+}