feat(ops): push action notifications to Axon agents over WebSocket

[05F3759DF]

Context

Axon Agent will poll Keystone for action requests as the reliable baseline, but online robots should receive low-latency notifications when an operator creates a new request. Keystone should push only notification metadata and keep the persisted action request as the source of truth.

Scope

• Push action.requested notifications to connected Axon Agents when a new action request is created.
• Include only request_id and minimal routing metadata in the WebSocket payload.
• Do not include raw commands, local script paths, or sensitive action arguments in the push payload.
• Record push delivery attempts for diagnostics.
• Ensure offline robots can still pick up requests later through polling.

Acceptance Criteria

• Online agents receive a WebSocket notification after a new action request is accepted.
• Offline agents do not lose requests because the request remains persisted for polling.
• Push delivery failures are visible for diagnostics but do not fail request creation.
• Push payloads do not contain arbitrary shell commands or sensitive values.
• Tests cover online delivery, offline fallback, duplicate notification tolerance, and push failure reporting.

Depends On

• feat(ops): create action request API with audit trail #66
• feat(ops): expose action execution status and logs #67

Related

• feat(agent): sync action capabilities and poll Keystone action queue axon#93
• feat(agent): receive Keystone WebSocket action notifications axon#94