Add `/resume -` shortcut to return to previous session

Feature Request

After using /fork to branch a conversation, there's no quick way to return to the original/parent session. You have to manually track and type the full session ID.

Proposal

Add a /resume - shortcut (analogous to cd - or git checkout -) that resumes the most recent previous session. This would make the fork/resume workflow much more fluid:

1. /fork → work on a tangent
2. /resume - → back to the original conversation

Alternatives considered

• /resume .. (parent notation) — but - is more familiar from shell conventions and works even when the previous session wasn't a parent (e.g., you just switched between two unrelated sessions)

Context

This is a small ergonomic improvement that would make /fork significantly more useful for exploratory workflows where you frequently branch and return.