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.

README.md

@@ -5,7 +5,7 @@
 <img src="docs/readme/lockup-dark.svg#gh-dark-mode-only" alt="Cobble" height="56">
 </h1>
 
-A crew of small AI worker bots you host yourself.
+A crew of small AI worker bots you host yourself (alpha).
 
 ![Cobble: a crew of small AI worker bots you host yourself](docs/readme/hero-light.png#gh-light-mode-only)
 ![Cobble: a crew of small AI worker bots you host yourself](docs/readme/hero-dark.png#gh-dark-mode-only)
@@ -19,6 +19,13 @@ A crew of small AI worker bots you host yourself.
 
 </div>
 
+> **Alpha.** Cobble is still in alpha: interfaces, database schema and
+> settings change between releases, and things break. Install it to try it
+> out and to tell us what is wrong, but keep your own backups (Settings >
+> Backups) and do not put it in front of work you cannot lose. Every release
+> is a real release: the installer updates an existing install in place and
+> keeps its data.
+
 Cobble is a self-hosted crew of small AI worker bots. You get a coordinator
 and a set of workers, each with its own job, models, memory and workspace.
 They talk to you in one chat, share workspaces, run routines on a schedule,
@@ -49,6 +56,7 @@ your data on your own machine.
 | Crew       | One coordinator and as many workers as you need, each with a name, a role, a hue and its own core instructions                                             |
 | Rail       | The coordinator, your recent workers and their latest sessions, then your workspaces, each row carrying its status line                                    |
 | Chat       | Markdown with @-mentions for bots and #-mentions for workspaces, replies and quotes, reactions, attachments, older messages on scroll, drafts kept locally |
+| Stop       | Stop a bot from the thinking row or the header menu: its queued and running work stops at once, everywhere you chat with it, and it goes idle              |
 | Header     | The bot's model, provider and computer at a glance, one click from either, and its picture, instructions and settings on the avatar                        |
 | Workspaces | A shared home for a group of bots: shared context, per-workspace instructions and memory, a default computer, and a dashboard                              |
 | Memory     | Each bot keeps core memory, memory about you, and workspace memory, and rewrites it as it works                                                            |