fix(dashboard): replace stale t.domain with t.domain_slug across all pages

After the v0.5 migration TopicRead.domain was renamed to domain_slug.
index.md, decisions.md and tasks.md still referenced the old field,
causing every workstream domain to fall back to "unknown". Also
updated tasks.md to load the domain filter list dynamically from
/domains/ instead of the hardcoded 6-slug array.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

state-hub/dashboard/src/decisions.md

@@ -24,7 +24,7 @@ const decState = (async function*() {
         data = decisions
           .map(d => ({
             ...d,
-            domain:       topicMap[d.topic_id]?.domain ?? null,
+            domain:       topicMap[d.topic_id]?.domain_slug ?? null,
             topic_title:  topicMap[d.topic_id]?.title  ?? null,
           }))
           .sort((a, b) => {