/clear should rotate session ID instead of truncating

Problem

/clear truncates the current session's history but keeps the same session ID. This means the session list doesn't reflect distinct work tasks — it's just "times I launched Claude."

There's no way to start a fresh session without quitting and restarting. In the desktop app and web app, this is especially friction-y since you can't just open a new terminal.

Proposed behavior

/clear (or a new /new command) should:

1. Archive/close the current session (preserving it in session history)
2. Start a new session with a fresh ID

This way, sessions map to tasks/topics, and /resume becomes a useful way to navigate past work.

Current workaround

Quit and restart Claude Code. The old session is preserved and resumable, but the restart is unnecessary friction.