Configurable exit resume hint

### Describe the feature or problem you'd like to solve
Since v1.0.64, the exit message's `copilot --resume=...` hint always shows the alphanumeric session ID, even for sessions that have been renamed (via `/rename` or auto-generated naming). This was an intentional fix (#3128) to avoid ambiguity between near-duplicate AI-generated titles, but it removed the option for users who deliberately rename sessions to something memorable and would prefer to see and reuse that name at exit. There's currently no setting to restore the previous behavior.

### Proposed solution
Add a user setting, e.g. `"exitMessage.resumeHint": "id" | "name" | "auto"`, that controls what the exit hint prints:
- `"id"` (default) - current behavior, always show the session ID. Preserves the reliability fix from #3128 for users who don't opt in.
- `"name"` - show the friendly/renamed name when the session has one, falling back to ID otherwise.
- `"auto"` - same heuristic the CLI used pre-1.0.64 (ID unless renamed).

This keeps the current default intact while letting users who prefer memorable, renamed resume hints opt back in.

### Example prompts or workflows
- User runs `/rename my-feature-work`, exits, and sees `copilot --resume=my-feature-work` instead of `copilot --resume=a1b2c3d4-...`.
- User sets `"exitMessage.resumeHint": "name"` once in `settings.json`; every future session exit reflects the friendly name when one exists.
- `/settings set exitMessage.resumeHint name` toggles it from within the CLI without hand-editing settings.json.
- Users who don't set the option see no change in behavior (still ID-only, per #3128).

### Additional context
Related discussion: #3128 ("Bring back the alphanumeric Session ID") - the maintainer response there (and the v1.0.64 changelog entry "Exit message always shows the session ID in the resume command instead of the friendly name") explains the reasoning for defaulting to ID. This request isn't to revert that default, just to make it configurable for users with the opposite preference.

先追蹤結束訊息如何建立其 copilot --resume 提示，以及如何讀取 settings.json 中的值。檢查 /settings 命令和 /rename 流程，以了解可用的工作階段名稱，然後驗證 id、name-with-ID-fallback 和 auto 行為。完成的標準是預設值仍為 ID-only，且新設定可透過 settings.json 和 /settings 兩者運作。

由索引模型根據 Issue 內容生成。