Initial commit: AutoTrader Marker Firefox extension

Mark AutoTrader listings as 'not wanted' on both search results and detail
pages. Includes the unlisted signing / auto-update pipeline (web-ext +
dist/updates.json polled by Firefox).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

dist/updates.json

@@ -0,0 +1,7 @@
+{
+  "addons": {
+    "autotrader-marker@tony.codes": {
+      "updates": []
+    }
+  }
+}