Desktop app ignores `askUser: false` from settings.json and offers no way to disable the ask_user tool

> *This was generated by AI during triage.*

## Summary

The desktop app has no way to disable the `ask_user` tool. The CLI setting `askUser: false` in `~/.copilot/settings.json` is read by the CLI entry point only; the app is a separate host that never consults it and exposes no equivalent toggle, so the same configuration directory yields different agent behavior depending on which client opens the session.

## Environment

- GitHub Copilot desktop app v1.0.26
- App-spawned Copilot CLI runtime v1.0.71
- Copilot CLI v1.0.75
- Windows 11
- `~/.copilot/settings.json` contains `"askUser": false`

## Reproduction

1. Set `"askUser": false` in `~/.copilot/settings.json`.
2. Start `copilot` in a terminal and confirm the agent has no `ask_user` tool.
3. Open the desktop app against the same configuration directory and start any chat or project session.
4. Observe that the agent still has `ask_user` and uses it.

## Expected behavior

One of the following, in order of preference:

- The app honors `askUser` from `~/.copilot/settings.json`, matching the CLI.
- The app exposes its own setting that maps to the SDK session option `askUserDisabled` (or `defaultAgent.excludedTools: ["ask_user"]`, which the app's own session wire already carries).
- The app documents that `askUser` is CLI-only, so the setting is not silently inert.

## Sanitized evidence

Read-only inspection of the installed binaries:

```text
CLI 1.0.75 app.js
askUserDisabled: (…) || settings.askUser === false # prompt mode
askUserDisabled: !B # interactive mode, B from --ask-user/--no-ask-user
option("--no-ask-user", "Disable the ask_user tool (agent works autonomously without asking questions)")

Desktop app 1.0.26 github.exe
SessionCreateWire fields: availableTools, excludedTools, defaultAgent, … # no askUserDisabled
string "askUser" — no occurrences
chat session tool set (Rust, src-tauri/src/session/core.rs region):
ask_user glob rg skill view web_fetch web_search
```

- `~/.copilot/data.db` tables `settings` and `app_state` hold no tool-permission or `askUser` state.
- The app does read `~/.copilot/config.json` and `~/.copilot/settings.json` (for example `skillDirectories`), so the file is in scope for the app; only this key is unhandled.

## Observed versus unconfirmed

**Observed:** `askUser: false` has no effect in the app, and no app setting, app-state key, or database column controls it.

**Unconfirmed:** whether a custom agent whose `tools:` allowlist omits `ask_user` fully suppresses the tool for a main app session — the SDK types describe `CustomAgentConfig.tools` as an allowlist, but this was not exercised end to end. Custom instructions can only discourage the tool, not remove it.

## Why it matters

Autonomy is a per-user working preference, not a per-client one. Users who deliberately run a no-questions agent lose that guarantee the moment the work moves from the terminal to the app, and the two clients disagree while reading the same configuration directory.

## Questions

1. Is `askUser` intended to remain CLI-only, or should host-independent settings apply to every client that reads `~/.copilot`?
2. Would `defaultAgent.excludedTools` be the preferred app-side mechanism, given the session wire already supports it?
3. Should settings the app cannot honor be surfaced as inert rather than ignored silently?

## Additional context

This was a read-only investigation: binaries, the configuration directory, and a copy of `data.db` were inspected; nothing was modified.

從 src-tauri/src/session/core.rs 開始，檢查桌面工作階段如何建構其工具集和 SessionCreateWire 欄位。將該路徑與所檢查的 app.js 證據中 CLI 對 askUser 的處理進行比較。完成的標準是：應用程式始終遵循 askUser: false，或提供並記錄等效設定，並驗證產生的工作階段不包含 ask_user。

由索引模型根據 Issue 內容生成。