feat(FN-3924): enforce blockedBy blocker invariants in scheduler and recove

Implements invariant enforcement for blocked-by relationships in the scheduler and self-healing recovery, preventing tasks from being blocked by stale or invalid dependencies. The feature adds tests for both scheduler and self-healing behavior, updates the CLI extension and architecture docs, and sh

Fusion-Task-Id: FN-3924

packages/cli/src/extension.ts

@@ -532,7 +532,7 @@ export default function kbExtension(pi: ExtensionAPI) {
       }
       if (params.agentId !== undefined) {
         const normalizedAgentId = normalizeNullableStringInput(params.agentId);
-        if (normalizedAgentId !== undefined && normalizedAgentId !== null) {
+        if (typeof normalizedAgentId === "string") {
           const error = await validateAssignableAgentId(ctx.cwd ?? process.cwd(), normalizedAgentId, task);
           if (error) {
             return {