feat(plugins): add Anki card-mined event, AnkiConnect invoke, anki-sync

- Add ctx.anki.on('card-mined') fed by polling, proxy, and card creation once enrichment finishes
- Add ctx.anki.invoke to send one AnkiConnect action to the effective url with no retries
- Ship example anki-sync plugin that syncs to AnkiWeb after mining goes quiet or playback ends
- Unload plugins that finish activating after unloadAll gave up on them
- Document the Anki API and anki-sync plugin in docs-site and architecture docs

assets/plugins/anki-sync/plugin.json

@@ -0,0 +1,5 @@
+{
+  "name": "Anki Sync",
+  "version": "1.0.0",
+  "main": "main.js"
+}