docs: document model lifecycle and deprecation

[mnkj0021]

Fixes #1275

Summary

• document how provider model scanners add, refresh, retire, and restore coding-agent models
• explain provider end-of-life metadata versus actual model disappearance
• document the 24-hour warning/grace window before unavailable models are rejected for new submissions
• explain default-model rotation and the effect on profiles, historical runs, and reproducibility
• add the guide to the docs sidebar and link it from the coding-agent guide

Validation

• cd website && corepack pnpm build — passed on a fresh recheck; 204 pages built and search index generated
• git diff --check — passed
• branch is current with upstream main (0 commits behind)

GitHub's upstream documentation/site workflows are still in action_required and need a Scope maintainer to approve the fork workflow run. The Microsoft CLA check passes.

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