Helpers on another model, chosen by logic rather than by the model

A helper still runs on the chat's own model by default. Other models are
designated per main model -- offered to it, or by hand only -- by the
instance or, with helpers.designate, by a person. subagent_run gains a
model argument whose enum is exactly the candidates that passed two
checks: capacity (a model that serves one request at a time cannot be
its own helper; a connection that holds one model at a time cannot serve
a helper on another of its models) and the model rules. A helper on
another model takes that model's own effort and data group. The composer
gains a Helpers picker; the model page, the connection form and Settings
gain their switches and lists.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

CHANGELOG.md

@@ -16,6 +16,43 @@ for 1.0.0 have something to be assembled from.
 
 ## Unreleased
 
+## 1.12.0
+
+Helpers on another model. This is the last of the three releases.
+
+- **A helper can run on a different model.** A helper is still the chat's own
+  model by default. On each model's page, **Helpers** lists other models this
+  one may send helpers to. Each is either *offered to the model*, so it can
+  choose that model itself, or *by hand only*, so it is used once somebody adds
+  it to a chat. When there is more than one choice, `subagent_run` gains a
+  `model` argument limited to exactly those models, and the model is told what
+  each is for. A helper on another model runs with that model's own default
+  reasoning effort, never the parent's, which it might refuse. It reads its own
+  data group's memories and notes.
+- **The composer's Helpers button** lists the models designated for the chat's
+  model. Tick one to add it to this chat, on the new-chat screen as well.
+- **Capacity is decided by logic, not by the model.** Two new switches, both off
+  by default, so nothing changes until they are set:
+  - **Serves one request at a time**, on a model. It cannot be its own helper,
+    because the helper would wait behind the reply that is waiting for it.
+  - **Holds one model at a time**, on a connection (llama-swap in front of one
+    GPU). Its models may still be their own helpers, but never send one to
+    another model on the same connection, because loading it would unload the
+    model whose reply is waiting.
+
+  A model with no helper it can use is no longer offered `subagent_run` at all,
+  instead of being offered a tool that refuses every call. Asking a friend and
+  the crowd are not affected: both are sequential, and the wait for a model to
+  load is accepted there.
+- **The model rules apply to helpers too.** A designation the model chooses
+  itself must be allowed by the rules. One added to a chat by hand needs what a
+  crowd member added by hand needs. A model in another data group needs a rule
+  that allows it.
+- **Your own helper models.** With the new *Choose their own helper models*
+  permission (off by default), Settings → Models lets a person designate helpers
+  for their models. These are added to the instance's, and a row of theirs for
+  the same pair replaces the instance's.
+
 ## 1.11.0
 
 Rules for which model may talk to which. This is the second of three releases;