New turn_handoff protocol for handing off in-progress turns to cloud workers

What

• New turn_handoff request and response let a session client transfer an in-progress turn (one whose last assistant message has tool calls that haven't run yet) to a worker process, which validates and runs the calls and continues the turn.
• The worker replies as accepted or duplicate, or with typed errors such as turn_handoff_disabled, invalid_handoff, unsupported_tool, or recovered_by_restart.
• A new turn_handoff_available system event is emitted once a worker can accept a handoff, carrying the tools it will accept, a worker_epoch, and an optional relay_marker flag, so a client learns the capability from the event stream instead of probing the worker.
• This applies to managed cloud worker sessions: admission is computed from sdkUrl and CLAUDE_CODE_REMOTE_SESSION_ID being set and CLAUDE_CODE_ENVIRONMENT_KIND being unset.
• A tengu_turn_handoff_run telemetry event was added to track handoff usage.

Why

This lets a turn keep going across a worker restart or handoff in managed cloud sessions, rather than losing an in-progress tool call.