A thread can now move to a linked environment with its conversation and its
git work, with exactly one copy live. ThreadHandoff marks it departing,
which refuses new turns and keeps queued runs from starting; packs the
branch and working tree with HandoffGit; uploads the bundle through the
other side's signed attachment route; and calls t3_thread_import there,
which now takes the bundle and applies it in a new worktree before creating
the thread. Only then is the thread departed here and read-only. A failed
move marks it failed, which takes turns again, and nothing is left there.

An agent can move its own thread with t3_thread_handoff. The move then
waits as pending until its turn ends, starts the thread there with the
agent's continuationPrompt, and is cancelled by a user message sent before
the turn ends. A startup sweep runs moves that a restart cut short; import
ids derive from the handoff, so a retry finds the thread already there.

threadHandoff.options / start / cancel serve the clients.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>