docs: update Claude Code subscription mode model ids and metrics

[inference-gateway-maintainer]

Summary

Refreshes the "Claude Code subscription mode" section of github-action.md after inference-gateway/cli#650 (closes inference-gateway/cli#649) changed the Claude Code model-id convention and started surfacing run metrics. Supersedes the now-stale parts of #276.

• Model ids: document anthropic/-prefixed ids as canonical (same convention as gateway mode and the pricing table), with bare Claude ids still accepted and normalized for back-compat. Removed the prior "must be a bare id / rejects provider-prefixed id" statement and noted that the CLI strips the provider prefix before invoking the claude CLI. Renamed the "Use bare Claude model ids" subsection to "Model ids".
• Run metrics: token usage (per assistant turn) and per-session cost now render in Claude Code mode, so the Tokens and Cost footer lines appear with no action-side change. Removed the "pending an upstream CLI fix / omitted for now" caveat.
• Added a note that cost is estimated from token counts via the CLI's pricing list (a subscription has no per-call billing), so it is an estimate of the equivalent pay-per-token price, not a billed subscription amount.
• Updated the model-id example in the enable snippet and the embedded example workflow to the anthropic/-prefixed canonical form.

Only github-action.md carries subscription-mode content; no other pages required changes. Verified with bun run lint:md, bun run format:check, and bun run build - all pass.

Closes #278