#785 schedule_goal crons collide: all share job_name agent_sched_<def_id[:8]> (fixed SCHEDULED_AGENT_DSL)

Same root cause as #784 but for schedule_goal (recurring crons). job_name = 'agent_sched_'+def_id[:8] where def_id is the FIXED SCHEDULED_AGENT_DSL definition, so EVERY schedule_goal (and pre-#784 every gmail watch) collides on one job_name. durable_cron is idempotent-by-job_name, so creating a 2nd schedule_goal silently no-ops (keeps the 1st goal) or clobbers it; you cannot have two recurring scheduled goals at once. FIX (same as #784): job_name = 'agent_sched_'+sha256(goal+'|'+cron)[:8] (unique per goal+cron; rescheduling identical params replaces) + delete-before-create (POST .../schedules/{job}/cancel, 404-tolerant) so a recreate registers fresh. stop_gmail_watch already handles the agent_sched_ prefix. MCP-only change (the hard-delete cancel endpoint shipped in #784).