Show session cost for claude.ai-authenticated enterprise users

Feature Request

Current behavior:
Session cost display in Claude Code CLI only works when billing flows through a direct Anthropic API key. Users authenticated via their claude.ai account (OAuth/enterprise) see no cost information in the CLI.

Expected behavior:
Enterprise users authenticated via claude.ai should see the same per-session cost/token usage display as API key users, either pulled from the claude.ai billing API or estimated locally using the same token-count method.

Why it matters:
Cost visibility is available on claude.ai for enterprise accounts, but there's no way to see it inline during a CLI session. This creates a disconnect for users who primarily work in Claude Code but want to track usage without switching to the browser.

Suggested approach:

• Expose a billing/usage endpoint on the claude.ai side that the CLI can query per session, or
• Apply the same local token-count cost estimation that API key users get, scoped to the enterprise pricing tier.