[chore](trx-rs): use nested SDK image path trx-rs/sdk

Match the image name that was pushed to the registry
(git.haxx.space/sjg/trx-rs/sdk) across the workflow, devcontainer and
README.

Assisted-By: Claude Code (claude-opus-4)
Claude-Session: https://claude.ai/code/session_01NFpGtGTWUEYXLwZeZs2RAV
Signed-off-by: Stan Grams <sjg@haxx.space>

.devcontainer/devcontainer.json

@@ -1,6 +1,6 @@
 {
   "name": "trx-rs SDK",
-  "image": "git.haxx.space/sjg/trx-rs-sdk:latest",
+  "image": "git.haxx.space/sjg/trx-rs/sdk:latest",
   "workspaceFolder": "/work",
   "workspaceMount": "source=${localWorkspaceFolder},target=/work,type=bind",
   "customizations": {