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

Failure modes

• Resume warns head_moved / files_diverged on your own branch — someone (possibly you, in another session) committed after the packet was created. Read the diff before trusting the packet's current_state; re-create the packet after material changes.
• memory: skipped with no_backend — the memory tier is unreachable from this process. The packet itself is unaffected; recall-based discovery of the handoff just will not fire.
• Cap rejections on create — the packet is a pointer, not a design doc. Move long prose into the spec or the branch's docs and reference it from the packet.
• A stale packet (packet_stale_days) — packets outlive their usefulness fast; treat one older than a week as archaeology, not instruction.