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

docs/grid-agent-control-plane.md

@@ -48,11 +48,11 @@ The JSON request envelope is stable and versioned. For example:
 {"version":1,"request_id":"health-1","request":{"method":"health"}}
 ```
 
-Observers can call `health`, `runtime`, `list_sessions`,
-`list_scheduled_jobs`, `list_pending_approvals`, `list_audit_events`, and
-`subscribe_events`. Operators can additionally call `cancel_request`,
-`pause_autonomy`, `resume_autonomy`, `cancel_action`, `decide_approval`,
-`force_reconnect`, `expire_conversation`, `set_roaming_job`,
+Observers can call `health`, `metrics`, `runtime`, `list_sessions`,
+`list_scheduled_jobs`, `list_pending_approvals`, `list_audit_events`,
+`list_observability_events`, and `subscribe_events`. Operators can additionally
+call `cancel_request`, `pause_autonomy`, `resume_autonomy`, `cancel_action`,
+`decide_approval`, `force_reconnect`, `expire_conversation`, `set_roaming_job`,
 `inject_operator_message`, and `graceful_shutdown`. Cancellation is a mutation
 and is operator-only. List requests use an opaque numeric cursor and a page
 size of 1 through 100.
@@ -62,6 +62,9 @@ region and pose fields when known, behavior mode, control-queue utilization,
 and aggregate budget use. Conversation responses contain metadata only. Audit
 and approval responses exclude arguments, prompt contents, credentials,
 authorization headers, capability URLs, model reasoning, and filesystem data.
+Unified events use pseudonymous correlation IDs and fixed-cardinality metrics;
+their schema, journal, redaction, and replay rules are documented in
+[`grid-agent-observability.md`](grid-agent-observability.md).
 `cancel_action` binds to the exact bounded action ID carried by behavior audit
 observations; it can cancel a queued or executing embodied action without
 preempting unrelated work. The built-in roaming job ID is `default-roaming`.