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/cgroups/iface.go

@@ -0,0 +1,14 @@
+package cgroups
+
+type ProcessType string
+
+const (
+	ProcessTypePTY   ProcessType = "pty"
+	ProcessTypeUser  ProcessType = "user"
+	ProcessTypeSocat ProcessType = "socat"
+)
+
+type Manager interface {
+	GetFileDescriptor(procType ProcessType) (int, bool)
+	Close() error
+}