Add toggle to show all sessions unscoped in `/resume` picker (bypass `cwd`/repo relevance grouping)

Problem

The /resume (and --resume) session picker scopes/groups sessions by relevance to the current working directory (branch/repo/git-root/directory, per #3277). In practice this means a session started in a different directory can be effectively invisible in the default picker list, even though it is very recent and fully resumable directly via /resume <session-id>.

Example:

• Session 11111111-2222-3333-4444-555555555555 (cwd C:\work\some-other-project, last updated same day) does not appear when running /resume from an unrelated cwd (C:\work).
• /resume 11111111-2222-3333-4444-555555555555 resumes it without any issue.

This is confusing: there is no indication that sessions are being filtered/deprioritized, so a session can appear "lost" when it's actually just scoped out of view.

Proposed solution

Add an explicit way to toggle off the directory/repo-based scoping in the /resume picker, so users can reliably see all local sessions regardless of cwd when needed. For example:

• A keybinding (similar to / for search, see #241) to toggle between "scoped" and "all sessions" view.
• And/or a flag, e.g. copilot --resume --all, to start directly in the unscoped view.

Why this matters

Relevance-based grouping is a nice default, but without an escape hatch it becomes an anti-feature: users lose confidence that /resume shows everything, and have to already know/remember a session ID to resume sessions from other directories.

Related

• #3277
• #241

Commencez par les points d’entrée du picker /resume et --resume, puis suivez le regroupement par pertinence décrit dans #3277 ainsi que l’interaction de recherche du picker référencée par #241. Déterminez si la modification doit utiliser un bouton bascule, un flag ou les deux, et vérifiez que la vue non limitée expose toutes les sessions locales tandis que la vue limitée existante reste la vue par défaut.