#758 Scheduling: MCP schedule_goal + durable_cron for recurring/scheduled agentic workflows

Added recurring scheduling via Highway durable_cron. New no-HITL scheduled-agent definition (agent_scheduled: goal via inputs, approval_required_tools=[]). MCP tool schedule_goal(goal, cron_expression): saves the agent def (idempotent by hash) + submits a durable_cron workflow (target_task_name=tools.workflow.execute, definition_id, target_params={inputs:{goal}}) that spawns the agent on the cron. durable_cron uses a chain pattern (each tick: register->sleep->spawn agent->spawn next iteration->complete) -> no history bloat, survives restarts, same mechanism Highway uses for its own internal crons. VALIDATED: every-minute cron spawned fresh unattended agent runs that fetched the live BTC price (1a7fcffd->64176; 6d0e64e0 at 01:28); stopped via workflow_schedules.is_active=false. schedule_goal verified: created daily '0 9 * * *' crypto-email job agent_sched_9b8a5ba1 (next_run 09:00 UTC, tenant demo). FOLLOW-UPS: cancel_schedule/list_schedules MCP tools + a real deactivation API (currently a workflow_schedules UPDATE); scheduled agents run UNATTENDED (no HITL) so schedule creation is the trust boundary.