Declare json-schema metadata decorators with auto dec

.chronus/changes/json-schema-auto-decorators-2026-9-2.md

@@ -0,0 +1,27 @@
+---
+changeKind: deprecation
+packages:
+  - "@typespec/json-schema"
+---
+
+The metadata-only decorators of this library are now declared as `auto dec`, so the compiler
+synthesizes their implementation and provides typed accessors. This affects `@baseUri`, `@id`,
+`@oneOf`, `@multipleOf`, `@contains`, `@minContains`, `@maxContains`, `@uniqueItems`,
+`@minProperties`, `@maxProperties`, `@contentEncoding`, `@contentMediaType`, `@contentSchema` and
+`@prefixItems`.
+
+Nothing changes for TypeSpec authors. For JavaScript consumers, the `$baseUri`-style implementation
+functions and their `BaseUriDecorator`-style signature types are deprecated: they are no longer what
+the compiler invokes. Use the generated `set*` accessor to apply a decorator programmatically:
+
+```ts
+// Before
+context.call($minContains, target, 2);
+
+// After
+import { setMinContains } from "@typespec/json-schema";
+setMinContains(program, target, 2);
+```
+
+Applying one of these decorators twice on the same declaration now reports a `duplicate-decorator`
+warning. The last application still wins.