Reduce latency and review-loop overhead in subagent workflows

Describe the feature or problem you'd like to solve

Subagent-driven implementation and review workflows can feel extremely slow. Agent startup, task handoffs, and repeated review/fix/re-review cycles often take minutes per round trip, making modest repository changes take much longer than expected.

Proposed solution

Reduce subagent startup and handoff latency, reuse context more efficiently between fix and review rounds, and collapse redundant review passes when prior findings have been addressed. Show clearer elapsed-time and current-work indicators so users can distinguish useful background work from a stalled workflow.

Example prompts or workflows

1. Implement a small configuration option plus regression coverage.
2. Run an implementer followed by a code-review agent.
3. Apply one review fix and re-review only the changed portion.
4. Complete a documentation-only task without another expensive full-context handoff.

Additional context

Observed with Copilot CLI 1.0.83 on Linux. The main concern is end-to-end latency and the number of sequential agent round trips, not model answer quality.

ファイル、テスト、実装のエントリーポイントは指定されていません。まず、CLI における subagent の起動、ハンドオフ、レビュー、修正／再レビューのワークフローを整理し、次に経過時間と現在の作業を示すインジケーターがどのように生成されるかを特定してください。完了の条件は、ラウンドトリップのオーバーヘッドが低減され、コンテキストが適切に再利用され、説明されているワークフローがカバーされていることです。