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/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,
+  }
+}