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

从 picker 入口 /resume 和 --resume 开始，然后追踪 #3277 中描述的相关性分组，以及 #241 中提到的 picker 搜索交互。确定此次更改应使用切换开关、flag，还是两者都使用，并验证未限定范围的视图会显示所有本地会话，同时现有的限定范围视图仍保持为默认视图。