Migrate remaining in-repo workstream REST callers to /workplans

Point cleanup_stale_tasks, validate_repo_adr, custodian_cli, dashboard data
loaders, extensions poll, MCP legacy resource, and consistency_check task
queries at preferred /workplans routes. Add caller inventory evidence doc.

dashboard/src/extensions.md

@@ -14,7 +14,7 @@ const epState = (async function*() {
     try {
       const [re, rw, rt, rr] = await Promise.all([
         apiFetch("/extension-points/"),
-        apiFetch("/workstreams/"),
+        apiFetch("/workplans/"),
         apiFetch("/topics/"),
         apiFetch("/repos/"),
       ]);