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_test.go

@@ -0,0 +1,15 @@
+package keys
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestSHA256Hashing(t *testing.T) {
+	t.Parallel()
+	hasher := NewSHA256Hashing()
+
+	hashed := hasher.Hash([]byte("test"))
+	assert.Regexp(t, "^\\$sha256\\$.*", hashed)
+}