Give a llama session a thinking level, asked of the model

A `thinking` param on the llama driver: "auto", "off", or a level, applied as a
chat-template argument on the next request -- `reasoning_effort`, or
`enable_thinking: false` for off -- so unlike the server flags it costs no
reload. It lands in the session settings dialog beside the other model
settings, which is what declaring it in `DriverKind::params` buys.

Which levels exist is the model's answer rather than a constant, because the
vocabularies disagree: the 27B here takes low, medium and xhigh and **raises**
on high and max, so a fixed list is a turn that fails on send. The driver asks
the loaded server (`thinking_options`) -- `chat_template_caps.
supports_reasoning_effort` for whether levels mean anything at all, which is
the gate that stops the control silently doing nothing on a template that
ignores the argument, then `/apply-template` per level, one cheap render each
at load time. Off is a separate argument and a separate question: honoured when
turning it off renders a different prompt, and both renders have to have
worked, since a template that refuses it also renders differently.

A level the loaded model cannot take is dropped from the request and said in
the transcript, naming what it does take. What is *not* said is anything about
a model nobody has asked yet: the answer is `Option<Vec<String>>`, where None
is "no server has been up" and an empty list is the model that genuinely takes
none.

Verified against the 27B on the GPU: "low" thought for 697ms and 79 characters,
"off" produced no thinking block at all, and "high" answered `this model does
not take "high" -- it takes off, low, medium, xhigh.` The picker wraps to two
rows in the settings dialog and shows the session's current value.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

AGENTS.md

@@ -80,6 +80,13 @@ Module-by-module intent is in PLAN.md's "Backend layout".
   14k), and as `running` it looked exactly like thinking. The phone draws
   both with the working spinner and its own words, "loading model" and
   "reading prompt".
+  **Thinking effort is a param, and which levels exist is the model's answer**
+  (2026-09-19): the `thinking` param rides on the request as a chat-template
+  argument (`reasoning_effort`, or `enable_thinking: false` for `off`), so it
+  needs no restart -- and the driver asks the loaded server which levels its
+  template actually takes rather than trusting the offered list, because the
+  27B raises on `high` and answers to `xhigh`. A level it cannot take is
+  dropped and said in the transcript, naming the ones it can.
   **Every one of those is a default rather than a constant** (2026-09-19):
   `DriverKind::params` declares what a provider takes — key, label, shape,
   and whether a change waits for a restart — and the phone renders whatever