[FEATURE]: Preemption primitives for sub-agent orchestration — max-turns-per-dispatch, sub-agent cancellation, per-tool-call streaming

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Describe the enhancement you want to request

We run opencode as the agent harness for a production multi-phase batch pipeline (openroadmap — a repo-wide march: many phases, each a separate opencode session, orchestrated by a parent). A recent no-op-loop post-mortem across 604 sessions found 208 (≈34%) were class-b no-op loops — a sub-agent repeating the same tool call (a re-read, a re-grep, a re-run) with no forward progress until a gate or manual intervention broke the cycle. The loops are cross-persona (qa 102, integration 9, archive 8, implementation 5–6).

The harness-side guardrails exist; the platform-side primitives do not. We request three platform capabilities so a harness can bound, cancel, and observe its sub-agents.

1. max-turns-per-dispatch — a hard per-dispatch turn budget

A per-subagent-Task-call maximum number of LLM/tool turns. When reached, the dispatch terminates with a distinct reason (max-turns-reached) surfaced to the parent (SDK event), partial transcript retained. The ceiling is settable by the orchestrator per dispatch, with a provider/operator default guard. AC intent: maxTurns: <N> on a dispatch; stops a loop at the bound without manual intervention or process kill.

2. Sub-agent cancellation — an operator/orchestrator-invoked cancel signal

A first-class cancel API on a specific dispatch that marks it cancelled at runtime level, terminates the child's LLM loop + pending tool call, propagates a terminal cancelled reason to the parent, and is idempotent for an already-parked/blocked dispatch. Distinct from a single tool-call interruption. AC intent: orchestrator can cancel a specific running subagent; parent reliably observes cancelled; works even when the child is parked on a permission ask / dropped stream / dead retry — no zombie sessions that block a parent turn forever.

3. Per-tool-call streaming — the tool-call output visible in-flight

Tool/output events stream to the session's event stream as the tool executes (partial output, progress, per-line for shell/exec), not only a single completion event, so a stalled or long-running call is visible and interruptible in-flight rather than surfaced only at completion. AC intent: a harness starts a long call and sees intermediate output before it completes; can interrupt it mid-flight; a stalled call is observable within a bounded window.

The three are independent and each independently addressable. Related open items: #17471 (output-token auto-continue — adjacent), #40175 (unbounded research fan-out — the unbounded-dispatch class P1 bounds), #27110 (max parallel sub-agents — a complementary concurrency ceiling). No client-side workaround is proposed — these require runtime-level guarantees.