Subagents can hand results back to their parent via a new SubagentHandback tool

What

• A new tool, SubagentHandback, lets a subagent send its final report straight to the agent (or main thread) that spawned it, as its one and only last tool call. It isn't a progress-update channel — it can only be called once, requires a non-empty message, and only works when the agent has an active "handback contract" with a recorded recipient.
• Long reports are compacted or summarized before delivery, and get safety-review framing (blocked/refused/unavailable) attached.
• The subagent's completion result now carries a handback payload (and a handbackInterim variant), and the finalize step computes whether the handback was sent, flagged, or withheld, appending a message to the result accordingly.
• Async subagents can now explicitly hand control back to a specific owning agent (tracked as handbackRecipient) rather than only running to completion or being polled.
• For subagents running in "auto" mode, this mechanism is what actually counts as the final answer: plain trailing text is discarded unless delivered through SubagentHandback. The feature is off by default, controlled by the CLAUDE_CODE_SENDMESSAGE_HANDBACK environment variable or an internal rollout gate.
• Depending on the handback outcome (send, flagged, or withheld), the step that classifies a sub-agent's final output is skipped or has its text discarded, so results aren't double-processed.

Why This gives subagents a reliable, explicit way to report back to whatever spawned them, instead of relying on trailing text that might get lost, mis-parsed, or double-classified — useful for orchestration flows where a parent agent needs a clean, single final report from a subagent it dispatched.