Deprecation headers, and a webhook when the changelog changes

Two ways to learn about a change without having to go looking for it.

• Deprecation headers. When an operation is being retired, every response it sends carries Deprecation (when it was deprecated), Sunset (when it stops working, once a date is set) and Link: <url>; rel="deprecation" pointing at the changelog entry that says what to use instead. Errors carry them too, so a client with a bad key still hears about it. No operation is deprecated today.
• `platform.changelog` webhook event. Subscribe an endpoint to it under Webhooks and it receives a signed event when a changelog entry written for agents is published, with the new entries, links to each, and the feed address. It goes only to endpoints that pick this event by name, never to one that takes every event. Each entry is sent to each endpoint once; it is not retried if the endpoint answers an error or times out, so read the changelog feed to catch up after your endpoint was down. In rare cases an entry can arrive twice, possibly under a different event id, so deduplicate on each entry's id in data.entries, not on the event id. Deliveries show in the endpoint's delivery log like any other.