[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>

.gitea/workflows/ci.yml

@@ -20,7 +20,7 @@ env:
 jobs:
   lint:
     runs-on: ubuntu-latest
-    container: git.haxx.space/sjg/trx-rs-sdk:latest
+    container: git.haxx.space/sjg/trx-rs/sdk:latest
     steps:
       - uses: actions/checkout@v4
       - name: rustfmt
@@ -30,7 +30,7 @@ jobs:
 
   test:
     runs-on: ubuntu-latest
-    container: git.haxx.space/sjg/trx-rs-sdk:latest
+    container: git.haxx.space/sjg/trx-rs/sdk:latest
     steps:
       - uses: actions/checkout@v4
       - name: Build