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/service_test.go

@@ -0,0 +1,14 @@
+package filesystem
+
+import (
+	"git.omukk.dev/wrenn/sandbox/envd/internal/execcontext"
+	"git.omukk.dev/wrenn/sandbox/envd/internal/utils"
+)
+
+func mockService() Service {
+	return Service{
+		defaults: &execcontext.Defaults{
+			EnvVars: utils.NewMap[string, string](),
+		},
+	}
+}