Session picker only shows current session (regression from ExP flight copilot_cli_remove_cwd_listing)

## Summary
The `/session` and `/resume` picker only lists the **current** session, hiding all other prior sessions for the same repository. Resuming by explicit ID (`/resume `) still works, so sessions are present on disk — only the listing UI is affected.

## Likely cause: ExP flight `copilot_cli_remove_cwd_listing`
My `~/.copilot/config.json` `expAssignmentsCache` has this A/B assignment enabled:

```
"copilot_cli_remove_cwd_listing": true
```

The regression began recently and correlates with this flight. This appears to be an unintended side effect of that experiment — it removes cwd/current-repo sessions from the default picker, leaving only the active session visible.

## Impact
- 55 local sessions on disk (39 for the current repo alone); the picker shows 1.
- Users lose the ability to browse/resume prior work from the UI and must recover session GUIDs manually from `~/.copilot/session-state/*/workspace.yaml`.

## Repro
1. Be assigned the `copilot_cli_remove_cwd_listing` flight.
2. Have multiple prior sessions in a git repo.
3. Run `/session` or `/resume` — only the current session appears.

## Workaround
`/resume ` by ID (GUIDs are the folder names under `~/.copilot/session-state/`).

## Request
Please treat the empty picker as a regression from `copilot_cli_remove_cwd_listing`, or provide a user-facing way to opt out / list all sessions regardless of cwd.

## Environment
- CLI version: 1.0.69
- OS: Windows (ARM64)
- Auth: gh CLI token

まず、割り当て copilot_cli_remove_cwd_listing を有効にした状態で picker の動作を再現し、その後 ~/.copilot/session-state/ 配下のセッションデータと ~/.copilot/config.json 内の割り当てを調べます。`/session` と `/resume` の一覧表示の動作を、明示的な ID による再開と比較します。現在のリポジトリの以前のセッションが、GUID を手動で復元しなくても表示され、選択できれば完了です。

索引モデルが issue の本文から書いたものです。