Cross-provider session handoff — verified packet create/resume so any agent can pick up a branch mid-flight

Notes & tips

• Delete the packet when its branch merges — the file is branch-scoped working state, not documentation.
• Fill verification rows with the probes you actually intend the receiver to run; the "not run" default keeps everyone honest.
• Both tools emit one structlog event each (handoff_create, handoff_resume) with the slug, warning codes, duration, and the memory outcome — provider-boundary usage shows up in telemetry reads.