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.

未指定任何文件、测试或实现入口。首先梳理 CLI 中 subagent 启动、交接、审查以及修复/重新审查的工作流，然后确定耗时和当前工作指示器是如何生成的。完成的标准是降低往返开销、适当复用上下文，并覆盖所述工作流。