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/execcontext/context.go

@@ -0,0 +1,37 @@
+package execcontext
+
+import (
+	"errors"
+
+	"git.omukk.dev/wrenn/sandbox/envd/internal/utils"
+)
+
+type Defaults struct {
+	EnvVars *utils.Map[string, string]
+	User    string
+	Workdir *string
+}
+
+func ResolveDefaultWorkdir(workdir string, defaultWorkdir *string) string {
+	if workdir != "" {
+		return workdir
+	}
+
+	if defaultWorkdir != nil {
+		return *defaultWorkdir
+	}
+
+	return ""
+}
+
+func ResolveDefaultUsername(username *string, defaultUsername string) (string, error) {
+	if username != nil {
+		return *username, nil
+	}
+
+	if defaultUsername != "" {
+		return defaultUsername, nil
+	}
+
+	return "", errors.New("username not provided")
+}