A thread moving here from a linked environment needs its conversation.
t3_thread_import creates the thread with that history as runless, imported
messages, so its first run gets it as a text context handoff whatever the
provider, then sends continuationPrompt as that run's message.

The thread id is a hash of the source environment, thread and handoff id,
and the continuation's command id derives from the thread, so a retried
import returns the thread the first created and starts nothing new. The
tool goes through startsThreads: modes are capped by the caller's, and a
link's import carries its linkOrigin. worktreePath, when given, must be one
of the project's worktrees.

The imported-history header no longer names "the previous T3 Code
orchestrator", since history now also arrives from other environments. The
capturing Codex adapter from the V1 cutover test moves to a testkit.

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