[FEATURE] Easier navigation between branched sessions

### Preflight Checklist

- [x] I have searched [existing requests](https://github.com/anthropics/claude-code/issues?q=is%3Aissue%20label%3Aenhancement) and this feature hasn't been requested yet
- [x] This is a single feature request (not multiple features)

### Problem Statement

When using /branch, Claude creates a new conversation and provides a session ID for returning to the original session via /resume .

This works, but switching back and forth between branched conversations requires copying or remembering session IDs. For short-lived branches or when frequently switching between two sessions, this feels unnecessarily cumbersome.

### Proposed Solution

Allow /resume to accept - as a shorthand for the previously active session:

/resume -

This could work similarly to cd - in a shell or git checkout -: it would switch back to the previously active session.

This would make navigating between branched conversations significantly more ergonomic without changing the existing session ID-based behavior.

### Alternative Solutions

None that come to mind.

### Priority

Medium - Would be very helpful

### Feature Category

CLI commands and flags

### Use Case Example

_No response_

### Additional Context

_No response_

Start by locating the /resume command and its handling of session IDs, then trace how /branch identifies the original session. Add support for '-' to switch to the previously active session while preserving explicit session-ID behavior. Done means /resume - returns to the prior session and the existing resume behavior still works.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.