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/spec/buf.gen.yaml

@@ -0,0 +1,14 @@
+version: v1
+plugins:
+  - plugin: go
+    out: ../internal/services/spec
+    opt: paths=source_relative
+  - plugin: connect-go
+    out: ../internal/services/spec
+    opt: paths=source_relative
+
+managed:
+  enabled: true
+  optimize_for: SPEED
+  go_package_prefix:
+    default: git.omukk.dev/wrenn/sandbox/envd/internal/services/spec