Expose remaining token count as env var for statusLine

Currently /context shows the remaining token count, but this value isn't available as an environment variable for use in the statusLine command. It would be useful to expose token usage metrics so users can display them in the status bar.

Proposed env vars:

• CLAUDE_CODE_TOKENS_REMAINING — remaining tokens in the context window
• CLAUDE_CODE_TOKENS_TOTAL — total context limit
• CLAUDE_CODE_TOKENS_USED — current token usage

Use case:

"statusLine": {
  "type": "command",
  "command": "echo \"📁 $(basename \"$PWD\") 📊 ${CLAUDE_CODE_TOKENS_REMAINING:-?}/${CLAUDE_CODE_TOKENS_TOTAL:-?}\""
}

Current workaround:
totalTokensReminder and /context work but aren't visible in the persistent status bar.