feat(steer): cascade steer to active child agents (subagent delegation)

[trac3r00]

[Bob] Propagate user mid-turn steers to active subagents.

Problem

When a user sends a steer while a subagent is running via delegate_task, the steer only reaches the parent agent. The parent is blocked waiting on the subagent, so the steer doesn't get delivered until after the subagent completes — making it useless for redirecting subagent work.

Solution

Mirror the existing interrupt() cascade pattern: when steer() is called on the parent, iterate _active_children under lock and call child.steer(cleaned) on each. The child's next tool result will include the OOB marker with the user's message.

• Recursive: grandchildren get it too
• Fault-tolerant: a broken child doesn't block others
• Thread-safe: uses existing _active_children_lock

Changes

• run_agent.py: +11 lines in steer() method
• tests/run_agent/test_steer.py: +71 lines (4 new tests)

Testing

All 39 steer-related tests pass.

[alt-glitch]

This was generated by AI during triage.

Related to #66046 for live subagent control, but this PR bundles that steer-cascade proposal with a separate CUA action-marker feature. Please split or rebase the independent changes so maintainers can evaluate the delegation behavior separately.

[trac3r00]

[Bob] Thanks for the review @alt-glitch — good call. I'll split out the CUA action-marker changes into a separate PR and rebase this one to focus solely on the steer-cascade delegation behavior. Will update shortly.

[teknium1]

Thanks for the focused steer-cascade implementation. The underlying gap is present on current main: run_agent.py:2864-2877 stores /steer only on the parent, while tools/delegate_tool.py:1392-1399 registers live children for the delegation lifetime.

Problems

• This PR still includes unrelated MoA tests from fa5480a47f08 (tests/cli/test_moa_one_shot_behavior.py and tests/tui_gateway/test_moa_one_shot_behavior.py, 398 lines). The member review requested that independent changes be split; the CUA portion was moved to #66618, but this MoA-only commit remains here.

Suggested changes

• Move the MoA test commit to the MoA-focused work and retain only the steer cascade and its focused tests (83b084d006d6).

Automated hermes-sweeper review.

[teknium1]

This new MoA-only test file is unrelated to the steer cascade. The prior member review requested splitting independent changes; please move this commit's MoA coverage to the MoA-focused work so this PR remains reviewable as the delegation change.

[teknium1]

Thanks for this — the gap was real (steer didn't cascade to children the way interrupt does). The steer-delivery slice just landed via #76805, which adds a targeted steer_subagent() registry primitive with delivered/queued receipts; the retained-subagent work now building on top of it (tracker #79686 P1) uses targeted per-child delivery rather than a blanket broadcast to all children, so the cascade approach here is superseded. If a broadcast-fallback mode proves useful once the registry lands, we'll credit this PR for the pattern. Closing — appreciated.