Reorganize worker observability layer into worker/observability/ (M8 part 3/3)

Collects the five post-run diagnostic + escalation modules into src/main/worker/observability/: run_events persistence, stdout/stderr noise filter, credential-shape log redactor, self-wake circuit breaker, and retry-aware failure escalation pipeline. circuit-breaker.ts and failure-escalation.ts remain as standalone files inside the directory — each owns a distinct state machine that deserves its own boundary — rather than collapsing into a single index.ts. handlers/*.ts and tests/unit/*.test.ts updated to point at the new paths.

tests/unit/self-wake.test.ts

@@ -1,5 +1,5 @@
 import { describe, expect, it } from "vitest";
-import { createCircuitBreaker, MAX_CONSECUTIVE_WAKES } from "@main/worker/circuit-breaker";
+import { createCircuitBreaker, MAX_CONSECUTIVE_WAKES } from "@main/worker/observability/circuit-breaker";
 
 /**
  * Tests for the self-wake circuit breaker used in worker-message-handler.ts.