feat(grid-agent): add structured observability and replay (#127)

crates/metacrate-grid-agent/README.md

@@ -57,3 +57,7 @@ and metadata-only operator controls are specified in
 The versioned integrated/TCP protocol, roles, framing, bounds, TLS remote-mode
 requirements, events, and management methods are specified in
 [`../../docs/grid-agent-control-plane.md`](../../docs/grid-agent-control-plane.md).
+The unified event schema, pseudonymous correlations, bounded JSONL rotation,
+fixed-cardinality metrics, diagnostic-capture warning, and non-executing replay
+contract are specified in
+[`../../docs/grid-agent-observability.md`](../../docs/grid-agent-observability.md).