Usage data is incorrect when sessions are resumed #524

Describe the bug

When you run /usage on a session you resumed, the usage data is incorrect.

The data is a mix of info from the current invocation of the CLI and the overall lifetime of the session (e.g. the wall clock time seems to be based on when I started the session, as well as the code changes, but the PPU usage and duration seem to be related to this launch).

Example:

Usage:
Total usage est:       0 Premium requests
Total duration (API):  0s
Total duration (wall): 7h 24m 54.139s
Total code changes:    375 lines added, 190 lines removed

Affected version

No response

Steps to reproduce the behavior

1. Create a new session and send a message
2. Exit the session
3. Resume the session
4. Run /usage

Expected behavior

Usage data should stay accurate across multiple resumes of the session.

Additional context

No response