Merge next into issue-16-exact-results-past-float64

Both sides added a line at the top of the Completed Steps in
docs/TODO.md; both are kept, this branch's first.

Model: opus-5-5

docs/TODO.md

@@ -29,6 +29,8 @@ with no deprecation warning.
 
 - 2026-09-29 Exact results past the range of a double, such as `2^1200`,
   are written to 17 significant digits instead of being refused
+- 2026-09-29 `POST`, `GET` and `DELETE` on a chat's webhooks, under
+  `/api/v1/chats/{id}/webhooks`, kept in `$DATA_DIR/webhooks.json`
 - 2026-09-29 `GET` and `POST /api/v1/chats/{id}/messages`: a chat's
   latest messages, and sending one
 - 2026-09-29 Added the HTTP API in `internal/api`: the server on `PORT`,