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

Start by reproducing the picker behavior with the copilot_cli_remove_cwd_listing assignment enabled, then inspect the session data under ~/.copilot/session-state/ and the assignment in ~/.copilot/config.json. Compare `/session` and `/resume` listing behavior with explicit ID resumption. Done means prior sessions for the current repository are visible and selectable without requiring manually recovered GUIDs.

Written by the indexing model from the issue text.