Feature: enterprise/org server-managed settings (incl. `env`) for the local Copilot CLI

### Problem
Org admins have no way to centrally push configuration — especially environment variables — to developers' **local** Copilot CLI installs. Today the only org-managed env mechanism is **Agents/Codespaces secrets**, which only reach GitHub-hosted **cloud** environments. For the local CLI (incl. VS Code dev containers on Colima/Lima/Podman), the only options are MDM-placed files or per-host provisioning — neither of which reaches into an unmanaged container.

### Request
A server-delivered, org/enterprise-managed settings layer for the local Copilot CLI — fetched at auth and refreshed periodically, taking precedence over user/repo settings — that can set a managed `env` block, e.g.:

```jsonc
// org-managed settings (delivered over the network, not a local file)
{ "env": { "MY_ORG_CONFIG_VALUE": "…" } }
```

These vars would apply to CLI sessions (and ideally subprocesses/plugin hooks), so org-distributed plugins/tools can pick up required config with **no MDM and no per-machine provisioning**.

### Why existing mechanisms don't cover it
- **Agents/Codespaces secrets** → cloud agent / Codespaces only; never reach the local CLI.
- **Org/enterprise policies** (#1971) → governance (enable/disable, model & MCP allowlists), not arbitrary config/secret delivery.
- **`.env` loading** (#2879) → local files the developer manages, not org-pushed.

### Prior art
Claude Code ships exactly this as **server-managed settings** (admin console → delivered at auth, refreshed hourly, reaches inside containers, supports an `env` block) — see code.claude.com/docs server-managed-settings. Enterprise-managed *models* already landed for Copilot CLI (#3730), so an enterprise-managed *settings/env* layer is a natural extension. Would also complement the Claude-parity work in #2471.

未指定實作檔案或測試。首先檢視 #3730 中現有的 enterprise-managed models 工作，以及相關的設定、原則和 .env issues（#1971 和 #2879），然後比較引用的 server-managed-settings 先例；完成的標準是就經過驗證的傳遞、重新整理、優先順序和受管理 env 的行為達成一致的設計。

由索引模型根據 Issue 內容生成。