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.

Start by tracing how the exit message builds its copilot --resume hint and how settings.json values are read. Check the /settings command and the /rename flow to understand the available session name, then verify the id, name-with-ID-fallback, and auto behaviors. Done means the default remains ID-only and the new setting works through both settings.json and /settings.

索引モデルが issue の本文から書いたものです。