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/multipart.go

@@ -0,0 +1,43 @@
+package utils
+
+import (
+	"errors"
+	"mime"
+	"mime/multipart"
+)
+
+// CustomPart is a wrapper around multipart.Part that overloads the FileName method
+type CustomPart struct {
+	*multipart.Part
+}
+
+// FileNameWithPath returns the filename parameter of the Part's Content-Disposition header.
+// This method borrows from the original FileName method implementation but returns the full
+// filename without using `filepath.Base`.
+func (p *CustomPart) FileNameWithPath() (string, error) {
+	dispositionParams, err := p.parseContentDisposition()
+	if err != nil {
+		return "", err
+	}
+	filename, ok := dispositionParams["filename"]
+	if !ok {
+		return "", errors.New("filename not found in Content-Disposition header")
+	}
+
+	return filename, nil
+}
+
+func (p *CustomPart) parseContentDisposition() (map[string]string, error) {
+	v := p.Header.Get("Content-Disposition")
+	_, dispositionParams, err := mime.ParseMediaType(v)
+	if err != nil {
+		return nil, err
+	}
+
+	return dispositionParams, nil
+}
+
+// NewCustomPart creates a new CustomPart from a multipart.Part
+func NewCustomPart(part *multipart.Part) *CustomPart {
+	return &CustomPart{Part: part}
+}