feat(core): emit SubagentProgress for child tool/turn milestones

[ZhiXiao-Lin]

Stacked on #35. Merge that one first; the base of this PR will retarget to `main` automatically once #35 lands.

Summary

The SubagentProgress event variant existed but was never emitted — the tracker from #35 had no mid-task signal to update from. This PR fills that gap so dashboards see meaningful intermediate state, not just Start → End.

Adds synthesize_subagent_progress() in tools/task.rs, called by the mpsc → broadcast forwarder inside TaskExecutor::execute_with_task_id. It translates two child-loop events into compact progress milestones on the parent broadcast:

• ToolEnd → status = \"tool_completed\", metadata = { tool, exit_code, output_bytes, error_kind? }
• TurnEnd → status = \"turn_completed\", metadata = { turn, total_tokens, prompt_tokens, completion_tokens }

Noisy events (TextDelta, ToolStart, ToolOutputDelta, nested Subagent*) are intentionally not translated — consumers needing token-level streaming should subscribe to the raw event stream.

The original child events are still forwarded unchanged; progress is additive.

What this unlocks

let snap = session.subagent_task(\"task-abc\").await.unwrap();
for entry in &snap.progress {
    println!(\"{} - {}\", entry.timestamp_ms, entry.status);
    // e.g. \"1731234567000 - tool_completed\" with metadata { tool: \"bash\", exit_code: 0 }
}
println!(\"last update: {}\", snap.updated_ms);

Test plan

• 4 new unit tests for synthesize_subagent_progress cover ToolEnd, ToolEnd with error_kind, TurnEnd, and the negative cases (TextDelta / ToolStart / TurnStart / nested SubagentStart).
• Extended parallel_task_executor_emits_subagent_events_for_each_child to assert at least 2 turn_completed progress events arrive on the broadcast for a 2-task parallel run.
• New integration test subagent_progress_events_accumulate_in_tracker verifies progress events flow through RuntimeEventSink into the tracker's progress Vec.
• cargo test --lib -p a3s-code-core: 1656 passed / 0 failed (4 new).
• cargo test --tests -p a3s-code-core: all integration binaries green.
• cargo clippy --lib --tests -p a3s-code-core: no new warnings.
• cargo fmt clean.

Out of scope (follow-ups)

• PR feat: SubAgent permissive mode and event streaming (v1.0.3) #3: Node SDK surface for subagentTask / subagentTasks / pendingSubagentTasks.
• PR Orchestrator does not execute custom agent definitions from agent_dirs #4: Python SDK alignment (includes the runs/run_events family).
• PR Orchestrator API: sub-agent event streaming and fine-grained permissive control #5: cancellation by task id (requires JoinHandle + CancellationToken tracking).