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/shared/smap/smap.go

@@ -0,0 +1,47 @@
+package smap
+
+import (
+	cmap "github.com/orcaman/concurrent-map/v2"
+)
+
+type Map[V any] struct {
+	m cmap.ConcurrentMap[string, V]
+}
+
+func New[V any]() *Map[V] {
+	return &Map[V]{
+		m: cmap.New[V](),
+	}
+}
+
+func (m *Map[V]) Remove(key string) {
+	m.m.Remove(key)
+}
+
+func (m *Map[V]) Get(key string) (V, bool) {
+	return m.m.Get(key)
+}
+
+func (m *Map[V]) Insert(key string, value V) {
+	m.m.Set(key, value)
+}
+
+func (m *Map[V]) Upsert(key string, value V, cb cmap.UpsertCb[V]) V {
+	return m.m.Upsert(key, value, cb)
+}
+
+func (m *Map[V]) InsertIfAbsent(key string, value V) bool {
+	return m.m.SetIfAbsent(key, value)
+}
+
+func (m *Map[V]) Items() map[string]V {
+	return m.m.Items()
+}
+
+func (m *Map[V]) RemoveCb(key string, cb func(key string, v V, exists bool) bool) bool {
+	return m.m.RemoveCb(key, cb)
+}
+
+func (m *Map[V]) Count() int {
+	return m.m.Count()
+}