feat: add useBoardPosts + useBoardChannels composables, remove useBoard

- 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

app/composables/useBoardPosts.js

@@ -0,0 +1,54 @@
+/**
+ * Board Posts Composable
+ * Shared state + CRUD for board posts.
+ */
+export function useBoardPosts() {
+  const posts = useState('board.posts', () => [])
+  const loading = useState('board.loading', () => false)
+
+  async function fetchPosts(params = {}) {
+    loading.value = true
+    try {
+      const result = await $fetch('/api/board/posts', { params })
+      posts.value = result || []
+      return posts.value
+    } finally {
+      loading.value = false
+    }
+  }
+
+  async function createPost(body) {
+    const created = await $fetch('/api/board/posts', {
+      method: 'POST',
+      body,
+    })
+    await fetchPosts()
+    return created
+  }
+
+  async function updatePost(id, body) {
+    const updated = await $fetch(`/api/board/posts/${id}`, {
+      method: 'PATCH',
+      body,
+    })
+    await fetchPosts()
+    return updated
+  }
+
+  async function deletePost(id) {
+    const result = await $fetch(`/api/board/posts/${id}`, {
+      method: 'DELETE',
+    })
+    await fetchPosts()
+    return result
+  }
+
+  return {
+    posts: readonly(posts),
+    loading: readonly(loading),
+    fetchPosts,
+    createPost,
+    updatePost,
+    deletePost,
+  }
+}