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_test.go

@@ -0,0 +1,74 @@
+package keys
+
+import (
+	"crypto/hmac"
+	"crypto/sha256"
+	"encoding/hex"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestHMACSha256Hashing_ValidHash(t *testing.T) {
+	t.Parallel()
+	key := []byte("test-key")
+	hasher := NewHMACSHA256Hashing(key)
+	content := []byte("hello world")
+	expectedHash := "18c4b268f0bbf8471eda56af3e70b1d4613d734dc538b4940b59931c412a1591"
+	actualHash, err := hasher.Hash(content)
+	require.NoError(t, err)
+
+	if actualHash != expectedHash {
+		t.Errorf("expected %s, got %s", expectedHash, actualHash)
+	}
+}
+
+func TestHMACSha256Hashing_EmptyContent(t *testing.T) {
+	t.Parallel()
+	key := []byte("test-key")
+	hasher := NewHMACSHA256Hashing(key)
+	content := []byte("")
+	expectedHash := "2711cc23e9ab1b8a9bc0fe991238da92671624a9ebdaf1c1abec06e7e9a14f9b"
+	actualHash, err := hasher.Hash(content)
+	require.NoError(t, err)
+
+	if actualHash != expectedHash {
+		t.Errorf("expected %s, got %s", expectedHash, actualHash)
+	}
+}
+
+func TestHMACSha256Hashing_DifferentKey(t *testing.T) {
+	t.Parallel()
+	key := []byte("test-key")
+	hasher := NewHMACSHA256Hashing(key)
+	differentKeyHasher := NewHMACSHA256Hashing([]byte("different-key"))
+	content := []byte("hello world")
+
+	hashWithOriginalKey, err := hasher.Hash(content)
+	require.NoError(t, err)
+
+	hashWithDifferentKey, err := differentKeyHasher.Hash(content)
+	require.NoError(t, err)
+
+	if hashWithOriginalKey == hashWithDifferentKey {
+		t.Errorf("hashes with different keys should not match")
+	}
+}
+
+func TestHMACSha256Hashing_IdenticalResult(t *testing.T) {
+	t.Parallel()
+	key := []byte("placeholder-hashing-key")
+	content := []byte("test content for hashing")
+
+	mac := hmac.New(sha256.New, key)
+	mac.Write(content)
+	expectedResult := hex.EncodeToString(mac.Sum(nil))
+
+	hasher := NewHMACSHA256Hashing(key)
+	actualResult, err := hasher.Hash(content)
+	require.NoError(t, err)
+
+	if actualResult != expectedResult {
+		t.Errorf("expected %s, got %s", expectedResult, actualResult)
+	}
+}