chore: remove dead artemis.debugMode setting

The `artemis.debugMode` config entry was never read anywhere in the
codebase — all debug/developer functionality is gated on
`artemis.developerMode` (as documented in CHANGELOG). The orphaned
entry only cluttered the VS Code settings UI without doing anything.

Author: Predixx

extension/package.json

@@ -177,11 +177,6 @@
           "default": false,
           "description": "Show the WebSocket connection status in the status bar during normal operation. When a connection failure occurs, the status bar item is always shown regardless of this setting."
         },
-        "artemis.debugMode": {
-          "type": "boolean",
-          "default": false,
-          "markdownDescription": "Enable debug mode to show additional developer tools like WebSocket status in the status bar. Useful for troubleshooting connection issues."
-        },
         "artemis.defaultCommitMessage": {
           "type": "string",
           "default": "Solution submission via Iris extension",