chore(devcontainer): wire commit message instructions and align jsonc formatting

- Link commit-messages.instructions.md via commitMessageGeneration.instructions
  setting so the VS Code SCM Generate button applies project commit rules
- Add explicit editor.formatOnSave: true to [jsonc] language block,
  matching the existing [json] block for consistent behavior

Release-Notes: skip
User-Impact: none

.devcontainer/devcontainer.json

@@ -79,10 +79,12 @@
         ],
         "[json]": {
           "editor.defaultFormatter": "esbenp.prettier-vscode",
+          "editor.formatOnSave": true,
           "editor.tabSize": 2
         },
         "[jsonc]": {
           "editor.defaultFormatter": "esbenp.prettier-vscode",
+          "editor.formatOnSave": true,
           "editor.tabSize": 2
         },
         "[python]": {
@@ -122,6 +124,11 @@
             "url": "${containerWorkspaceFolder}/schemas/json/translation_schema.json"
           }
         ],
+        "github.copilot.chat.commitMessageGeneration.instructions": [
+          {
+            "file": ".github/instructions/commit-messages.instructions.md"
+          }
+        ],
         "git.useConfigOnly": false
       }
     }