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/utils/atomic.go

@@ -0,0 +1,27 @@
+package utils
+
+import (
+	"sync"
+)
+
+type AtomicMax struct {
+	val int64
+	mu  sync.Mutex
+}
+
+func NewAtomicMax() *AtomicMax {
+	return &AtomicMax{}
+}
+
+func (a *AtomicMax) SetToGreater(newValue int64) bool {
+	a.mu.Lock()
+	defer a.mu.Unlock()
+
+	if a.val > newValue {
+		return false
+	}
+
+	a.val = newValue
+
+	return true
+}