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/services/filesystem/stat.go

@@ -0,0 +1,29 @@
+package filesystem
+
+import (
+	"context"
+
+	"connectrpc.com/connect"
+
+	"git.omukk.dev/wrenn/sandbox/envd/internal/permissions"
+	rpc "git.omukk.dev/wrenn/sandbox/envd/internal/services/spec/filesystem"
+)
+
+func (s Service) Stat(ctx context.Context, req *connect.Request[rpc.StatRequest]) (*connect.Response[rpc.StatResponse], error) {
+	u, err := permissions.GetAuthUser(ctx, s.defaults.User)
+	if err != nil {
+		return nil, err
+	}
+
+	path, err := permissions.ExpandAndResolve(req.Msg.GetPath(), u, s.defaults.Workdir)
+	if err != nil {
+		return nil, connect.NewError(connect.CodeInvalidArgument, err)
+	}
+
+	entry, err := entryInfo(path)
+	if err != nil {
+		return nil, err
+	}
+
+	return connect.NewResponse(&rpc.StatResponse{Entry: entry}), nil
+}