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/failure-escalation.test.ts

@@ -3,7 +3,7 @@ import {
   handleFailureEscalation,
   handleSuccessReset,
   type EscalationDeps,
-} from "@main/worker/failure-escalation";
+} from "@main/worker/observability/failure-escalation";
 
 /**
  * Tests for the retry-aware failure escalation pipeline.