Feature request: add a model retirement / end-of-life date field

Summary

The schema currently tracks several dates for a model but has no field for when a model is retired / shut down / no longer served by the provider — i.e. the last date the model is available.

Looking at packages/core/src/schema.ts, a model has:

• knowledge — knowledge cutoff
• release_date — first public release
• last_updated — most recent update
• status — enum (alpha | beta | deprecated)

So we can mark a model as deprecated, but there is no way to record the actual date it becomes (or became) unavailable.

Why this matters

Many models have a published retirement / shutdown date well before they actually stop working, and consumers of the API need to know it:

• openai/gpt-3.5-turbo and other legacy OpenAI models have announced shutdown dates.
• Knowing the end-of-life date lets tools warn users, hide unavailable models, or plan migrations.

Today there's no machine-readable way to express "this model stops being available on YYYY-MM-DD".

Proposal

Add an optional date field, e.g. retirement_date (or end_of_life / available_until), following the same ^\d{4}-\d{2}(-\d{2})?$ format used by release_date and last_updated:

name = "GPT-3.5-turbo"
release_date = "2023-03-01"
last_updated = "2023-11-06"
status = "deprecated"
retirement_date = "2024-XX-XX"   # date the model is no longer served

Happy to open a PR for the schema change + README docs if the maintainers agree on the field name.

Start in packages/core/src/schema.ts and inspect the existing release_date and last_updated definitions, including their date format, then check the README documentation mentioned in the proposal. Confirm the field name and optional-date semantics with maintainers; done means the schema accepts the retirement date consistently and the README documents it.