Feature Request: Configurable system prompt - allow users to slim down fixed token overhead

### Problem

The Copilot CLI system prompt consumes ~20,500 tokens at session start before any user content is loaded. On a 200K context window, that's ~10% consumed by instructions the user may not need. Combined with tool definitions (~8,500 tokens for tool usage guidelines alone), a significant portion of the context window is unavailable for actual work.

For comparison, [pi](https://github.com/badlogic/pi-mono) achieves robust agent behavior with a system prompt around ~1,000 tokens — roughly 20x smaller. I understand Copilot CLI's system prompt is intentionally comprehensive to ensure consistent, high-quality behavior across diverse user skill levels and use cases, and that's a reasonable design choice. But for power users in long sessions with large codebases, the overhead is noticeable.

### Proposal

Allow users to configure which system prompt sections are loaded, similar to how `--disable-builtin-mcps` lets users opt out of MCP servers. Ideas:

1. **`--slim-prompt` flag** — Load a minimal system prompt (core instructions only, skip verbose tool guidelines)
2. **`--disable-prompt-sections` flag** — Selectively disable sections (e.g., `--disable-prompt-sections plan-mode,git-rules`)
3. **Tiered prompts** — `--prompt-level minimal|standard|full` with increasing detail
4. **Lazy tool guidelines** — Load tool usage guidelines on first use of that tool rather than at session start (highest-impact single change)

### Impact

For power users who already know the tool conventions, trimming verbose guidelines could save 5,000-8,000 tokens — meaningful headroom for longer sessions and larger codebases. This would let Copilot CLI preserve its comprehensive defaults while giving experienced users an opt-in path to leaner sessions.

### Environment
- Copilot CLI v1.0.22
- Claude Opus 4.6
- Windows 11

未指定檔案、測試或實作入口。首先定位 Copilot CLI 組裝系統提示詞和工具指南的位置，然後釐清應實作所提議的哪種設定模型。完成內容應包括一種 opt-in 方式，以減少提示詞開銷，同時保留預設行為。

由索引模型根據 Issue 內容生成。