feat(plugins): add versioned plugin API with media snapshots and provena

- Add standalone src/plugin-api.ts types and manifest apiVersion check
- Add ctx.media playback snapshots, ctx.signal, ctx.onDispose, and awaitable ctx.mpv.request
- Bound async activation; actions wait only for their own plugin
- card-mined now carries per-note provenance (media + subtitle captured at mining start) through merges and YouTube queues
- Port episode-tags and auto-jimaku to provenance and media snapshots; drop mining-goal docs

assets/plugins/ai-explain/plugin.json

@@ -1,5 +1,6 @@
 {
   "name": "AI Explain",
   "version": "1.0.0",
+  "apiVersion": 1,
   "main": "main.js"
 }