Fix: enable discussion and discussion_comment events in app manifest

[m1lo9]

Problem

Handlers for discussion badges (handleDiscussionAddBadge, handleDiscussionCommentAddBadge) were already implemented in src/handlers/badges.ts and registered in src/index.ts, but the corresponding events were commented out in app.yml, so the app never received these webhooks.

Fix

Uncommented the discussion and discussion_comment events in app.yml so they match the handlers that already exist in the codebase.

Note for maintainer

This PR only updates the app.yml manifest file, which serves as a reference/template — as noted in the file's own comment, changing it does not automatically update the live GitHub App configuration. To actually enable these handlers in production, please also manually enable the discussion and discussion_comment events under Settings → Permissions & events → Subscribe to Events on the app's settings page, then click Save changes.