Stop allowing credentialed cross-origin requests (closes #261)

CORS() allowed every origin with credentials, so a page on another
origin of the same site could read what the signed-in user sees. It
now uses the options of the CONVENTIONS.md template: every origin as
*, no credentials, and the template's request headers. corsMaxAge
stays, since AGENTS.md asks for named constants.

Model: opus-5-5

README.md

@@ -168,8 +168,10 @@ either.
 - Request bodies for `POST /api/v1/session`, `POST /api/v1/login` and
   `POST /api/v1/messages` are limited to `MAX_MESSAGE_SIZE` bytes (default
   `4096`). A larger body gets `400` `invalid request body`.
-- The server allows cross-origin requests from any origin, with credentials. The
-  cookie is `SameSite=Strict`, so a browser sends it only from the same site.
+- A response to a cross-origin request carries `Access-Control-Allow-Origin: *`,
+  never the request's origin, and never `Access-Control-Allow-Credentials`. A
+  page on another origin therefore cannot read the response to a request that
+  carries the cookie.
 
 ### The auth cookie
 
@@ -1896,10 +1898,10 @@ The IRC listener has no TLS.
   carries a `Content-Security-Policy` header:
   `default-src 'self'; script-src 'self'; style-src 'self'; connect-src 'self'; img-src 'self'; font-src 'self'; object-src 'none'; frame-ancestors 'none'; base-uri 'self'; form-action 'self'`.
   The web client works within it.
-- Cross-origin requests are allowed from any origin, with credentials. The
-  allowed methods are `GET`, `POST`, `PUT`, `DELETE` and `OPTIONS`. The allowed
-  request headers are `Accept`, `Content-Type` and `X-CSRF-Token`. The cookie's
-  `SameSite=Strict` still keeps it to the same site.
+- A response to a cross-origin request carries `Access-Control-Allow-Origin: *`,
+  never the request's origin, and never `Access-Control-Allow-Credentials`. A
+  page on another origin therefore cannot read the response to a request that
+  carries the cookie.
 
 ### Logging