Sort /resume sessions by last activity time instead of creation time

Problem

When using /resume, sessions are sorted by creation time (first prompt). This means a session that was started a long time ago but was actively used until just now gets buried at the bottom, while a session started recently but barely used appears at the top.

Expected Behavior

Sort sessions by last activity time (most recent prompt/interaction) instead of session creation time.

Use Case

• I have a long-running session that I've been using heavily over several days
• I also have newer sessions that I opened briefly and abandoned
• When I type /resume, the long-running active session is hard to find because it was created days ago
• The abandoned sessions appear first because they were created more recently

Proposed Solution

Sort the /resume session list by the timestamp of the most recent message/prompt in each session, descending (most recently active first).