feat(steer): peer + broadcast session steering (session.steer_peer / steer_broadcast)

[HermesZum]

Summary

Adds peer-to-peer and broadcast steering between live sessions, on top of Hermes' existing steer_subagent child path and the generic AIAgent.steer (which appends to the last tool result and preserves the message-role invariant). New steer_session (peer) + steer_broadcast (peers + delegated children) in tools/delegate_tool.py, exposed as gateway methods session.steer_peer / session.steer_broadcast. Ports prime-agent's agent_message.send / peer-broadcast messaging by extending existing steering + delegation primitives — no architecture fork, no prompt-cache or message-role invariant break.

Closes #90287.

Changes

• tools/delegate_tool.py: steer_session (peer, resolver-injected, plugin-safe) + steer_broadcast (fans out to all live peer sessions + active delegated children). Child path reuses the existing steer_subagent unchanged — no new steering race, no new shared state.
• tui_gateway/methods_session.py: session.steer_peer + session.steer_broadcast gateway methods; authority mirrors subagent steering via the invoking session's exact steer triple.
• tests/tools/test_subagent_steer_f3.py (new, 8 tests): peer steer, unknown/no-resolver/empty safety, broadcast to peers+children, sender exclusion, no-op.

Design notes

• Reuses AIAgent.steer for the actual injection — appends to the target's last tool result, so no new user turn is created and the role-alternation invariant holds.
• Child steering delegates to the existing steer_subagent (same lock + accepting_steer gate), so there is no new race and no behavior change for child steering.
• Peer resolution is resolver-injected and plugin-safe (no hard-coded core special-casing beyond the thin gateway method).
• Opt-in / off by default: existing sessions and steering callers are unchanged; only the new gateway methods enable peer/broadcast.

Relationship to existing work (Step-0 duplicate search, see #90287)

• feat: expose subagent lifecycle tools (list_agents, kill_agent) + asy… #20846 / feat(delegation): add live subagent control #66046 (merged) cover subagent lifecycle (list_agents/kill_agent, live control). This adds steering (inject a directive into a live session), which those do not provide. No duplication.
• No in-flight effort covers peer steer or broadcast. Additive.

Test plan

• New tests/tools/test_subagent_steer_f3.py: 8 tests (peer steer, unknown/no-resolver/empty safety, broadcast to peers+children, sender exclusion, no-op).
• Regression: delegation session-lifecycle suite (12) green; delegate_tool.py + methods_session.py py_compile clean.

Invariants preserved

• Reuses AIAgent.steer → no new user turn, no role-alternation violation.
• Child path reuses steer_subagent lock + gate → no new race.

Notes / asks

• F3 only, split out from a larger prime-agent port to keep review surface small. F1 is PR feat(refine): reversible /refine undo (snapshot-before-write rollback) #90284; F2 (goal token budget) is PR feat(goals): opt-in token budget for /goal (input/output caps) #90285.
• Open question (in feat(steer): peer + broadcast session steering (session.steer_peer / steer_broadcast) #90287): gateway method namespace; whether cross-session steer should be gated behind a config flag.

[HermesZum]

Superseded by #90340 — the complete F3 (user entry point: name=, list_subagents(), steer_subagent_by_name(), delegate_task steer/by-name + steer_peer/broadcast actions, and session.steer_peer/session.steer_broadcast RPCs) on a single branch off current upstream/main. Closing this partial in favor of #90340.

[HermesZum]

Superseded by #90340 (complete F3).

[HermesZum]

Superseded by #90340 — the complete F3 implementation (user entry point: name=, list_subagents(), steer_subagent_by_name(), agent-facing control plane + gateway RPCs) now lives on a single branch off current upstream/main. Closing this partial plumbing PR in favor of #90340.