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

Comparison

• vs. the handoff contract file alone — the collaboration contract already tracks docs/handoffs/<branch-slug>.md as a hand-written file. Session-handoff keeps that location and template but makes the facts machine-derived and re-checkable: a receiving agent no longer has to trust that the listed SHA or file set is current.
• vs. session memory (session_memory_*) — the stash carries small cross-session findings; a handoff packet carries one branch's full working state. They link: create stashes a pointer so recall surfaces the handoff, but the packet file is the artifact.
• vs. /spec documents — a spec captures multi-session design intent; a packet captures a moment: this branch, this HEAD, this next action.