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/filesystem/model.go

@@ -0,0 +1,30 @@
+package filesystem
+
+import (
+	"os"
+	"time"
+)
+
+type EntryInfo struct {
+	Name          string
+	Type          FileType
+	Path          string
+	Size          int64
+	Mode          os.FileMode
+	Permissions   string
+	UID           uint32
+	GID           uint32
+	AccessedTime  time.Time
+	CreatedTime   time.Time
+	ModifiedTime  time.Time
+	SymlinkTarget *string
+}
+
+type FileType int32
+
+const (
+	UnknownFileType   FileType = 0
+	FileFileType      FileType = 1
+	DirectoryFileType FileType = 2
+	SymlinkFileType   FileType = 3
+)