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/useBoard.js

@@ -1,6 +0,0 @@
-export const useBoard = () => {
-  const getSuggestions = (params = {}) =>
-    $fetch('/api/board/suggestions', { params })
-
-  return { getSuggestions }
-}

app/composables/useBoardChannels.js

@@ -0,0 +1,33 @@
+/**
+ * Board Channels Composable
+ * Shared state + helpers for mapping board tags to Slack channels.
+ */
+export function useBoardChannels() {
+  const channels = useState('board.channels', () => [])
+
+  async function fetchChannels() {
+    const result = await $fetch('/api/board/channels')
+    channels.value = result || []
+    return channels.value
+  }
+
+  function resolveTagChannel(tagSlugs = []) {
+    if (!tagSlugs?.length) return null
+    return (
+      channels.value.find((channel) =>
+        (channel.tagSlugs || []).some((slug) => tagSlugs.includes(slug))
+      ) || null
+    )
+  }
+
+  function slackUrl(channelId) {
+    return `https://gammaspace.slack.com/archives/${channelId}`
+  }
+
+  return {
+    channels: readonly(channels),
+    fetchChannels,
+    resolveTagChannel,
+    slackUrl,
+  }
+}

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,
+  }
+}