feat: remove legacy OpenCode runtime

Cut product flows over to Coding/Pi and retain only the migration-owned v1 boundary. Promote supported native optional packages because electron-builder omitted pnpm transitive optional closure from the packaged ASAR.

pnpm-workspace.yaml

@@ -6,7 +6,6 @@ allowBuilds:
   koffi: true
   msgpackr-extract: true
   node-llama-cpp: true
-  opencode-ai: true
   protobufjs: true
   sharp: true