fix: enforce explicit workspace choices across update paths

Fail closed for an unavailable configured source, invalidate superseded workspace intent atomically on ordinary task updates, and reuse canonical workspace eligibility in channel settings.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

ui/src/api/execution-workspaces.ts

@@ -59,6 +59,7 @@ export const executionWorkspacesApi = {
       issueId?: string;
       status?: string;
       reuseEligible?: boolean;
+      selectableForTask?: boolean;
     },
   ) => {
     const params = new URLSearchParams();
@@ -67,6 +68,7 @@ export const executionWorkspacesApi = {
     if (filters?.issueId) params.set("issueId", filters.issueId);
     if (filters?.status) params.set("status", filters.status);
     if (filters?.reuseEligible) params.set("reuseEligible", "true");
+    if (filters?.selectableForTask) params.set("selectableForTask", "true");
     params.set("summary", "true");
     const qs = params.toString();
     return api.get<ExecutionWorkspaceSummary[]>(