Add Tasks dashboard page

New page with:
- Data fetch: /tasks/ + /workstreams/ + /topics/ in parallel, enriched
  with domain and workstream_title per task
- Task Overview KPI card in TOC sidebar: open / blocked (red if >0) /
  in progress / done with % of total
- Status Distribution chart (horizontal bar, colour-coded by status)
- Blocked Tasks section: cards with priority badge, domain, workstream,
  blocking_reason highlighted in amber
- All Tasks: filterable table (status, priority, domain, assignee
  multiselect + text), sorted blocked→in_progress→todo→done, 25 rows

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

dashboard/observablehq.config.js

@@ -4,6 +4,7 @@ export default {
   pages: [
     { name: "Overview", path: "/" },
     { name: "Workstreams", path: "/workstreams" },
+    { name: "Tasks", path: "/tasks" },
     { name: "Decisions", path: "/decisions" },
     { name: "Progress", path: "/progress" },
     {