feature: add /codex:usage command to show rate limits and account plan

[gignac-cha]

Summary

• Add a new /codex:usage slash command that displays the current account's rate limit status
• Read ~/.codex/auth.json and call the Codex backend usage API (/wham/usage) to fetch rate limits
• New command file: plugins/codex/commands/usage.md
• New handler: handleUsage in codex-companion.mjs
• New render functions: renderUsageReport and formatUsageWindow in lib/render.mjs
• Update tests/commands.test.mjs to include usage.md in the expected command list
• Add /codex:usage section to README.md

Problem

There is currently no way to check Codex rate limits from within Claude Code. Users must switch to the Codex TUI and run /status to see how much of their 5h or weekly limit remains.

Example output

# Codex Usage

Plan: Plus

Limits:
- 5h limit: 73% left (resets 1 Apr, 18:22)
- Weekly limit: 54% left (resets 5 Apr, 09:15)
- Code review Weekly limit: 91% left (resets 7 Apr, 14:30)

Implementation notes

• The Codex CLI does not currently expose a codex usage subcommand, so the plugin reads ~/.codex/auth.json directly and calls the usage endpoint. The API path (/wham/usage) was identified from the open-source Codex CLI codebase.
• --json flag outputs the raw API response for structured consumption.
• Handles expired tokens (401/403) with a hint to re-authenticate.

Testing

• Verified usage and usage --json output manually
• Existing test suite passes (the commands.test.mjs hardcoded list is updated)

Fixes #102

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This PR was authored with the assistance of Claude Code (Anthropic).