Implement /cost command and track session usage

Summary:
Implement a /cost command in OpenAI Codex, modeled after the similar command in Claude Code. This command should display a summary of resource usage and changes for the current session.

Description

When a user runs /cost, the CLI should output a summary containing:

• Total cost of the session so far (in USD)
• Total API duration
• Total wall-clock duration
• Total code changes (number of lines added, removed, and net change)

The output should be formatted similarly to:

/cost
⎿ Total cost:            $0.1524
⎿ Total duration (API):  23.7s
⎿ Total duration (wall): 1m 0.1s
⎿ Total code changes:    0 lines added, 0 lines removed

Acceptance Criteria

• [ ] Add a /cost command to the Codex CLI.
• [ ] Command outputs a formatted summary as shown above.
• [ ] Tracks and displays the total cost of API calls made during the session.
• [ ] Shows total duration spent using the API (aggregated).
• [ ] Displays real (wall-clock) session duration.
• [ ] Tracks and reports all code changes, including lines added and removed.
• [ ] No actual code changes should be made directly by this command.
• [ ] Proper error handling for cases when any summary data is missing.

Notes

• Use the Claude Code /cost command output as a reference for the user interface.
• The feature should only display information; it does not modify state.
• Please ensure performance impact is minimal when collecting and reporting data.