Delete uses findOneAndDelete with author match (no TOCTOU window);
existence check only runs on miss to distinguish 403 vs 404. Posts
list capped at 200. Drop unused resolveTagChannel and refreshParams;
route slack URL building through the composable's slackUrl helper.
- useBoardPosts: CRUD with useState('board.posts','board.loading')
- useBoardChannels: fetch + resolveTagChannel + slackUrl helpers
- useBoard.js removed (old suggestions wrapper); only app/pages/board.vue still imports it, will be rewritten in Phase 5
