Let a user stop a bot, and say the app is still alpha

The thinking row and the header menu both carry Stop while a bot has work
open for the user anywhere: every pending or processing job of that bot in
their conversations is failed as stopped, its task closed, a Stopped step
recorded and one closing message posted per session, so the thread stops
waiting and the bot goes idle. The handler running the job is aborted from
this process through a registry, so a job stuck waiting on a model stops at
once instead of on its next heartbeat, and a late face change after the
abort is dropped.

The release stage is now said in the app too: Settings > General reports
"Cobble v0.1.5 (alpha)" next to development, and the README and RELEASING.md
say what alpha means for installs.

RELEASING.md

@@ -3,6 +3,12 @@
 Cobble ships as a multi-arch container image and a single `install.sh`.
 The repository starts private and is made public at the launch gate below.
 
+Cobble is in **alpha** (see the note in `README.md`). Every release published
+before v1.0.0 is an alpha release: interfaces, the database schema and the
+settings may change between them, so a release that needs a migration step
+has to say so in its notes, and Settings > General keeps saying the stage
+until v1.0.0.
+
 ## One-time setup
 
 - [ ] `GITEA_USERNAME` and `GITEA_TOKEN` are set as GitHub Actions secrets.