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.

src/main/worker/handlers/run-log-handler.ts

@@ -1,5 +1,5 @@
-import { filterRunLogChunk } from "../run-log-filter";
-import { persistRunEvent } from "../run-event-recorder";
+import { filterRunLogChunk } from "../observability/run-log-filter";
+import { persistRunEvent } from "../observability/run-event-recorder";
 import type { HandlerContext, HandlerDeps, WorkerMessageEvent } from "./handler-context";
 
 const LEASE_RENEWAL_INTERVAL_MS = 5 * 60 * 1000; // 5 minutes