Render one assistant message per turn, and make the request timeout a setting

Two setup failures found while calibrating the new benchmark tasks,
both of which would also fail real milestones.

Devstral's chat template (Mistral) rejected any turn in which the model
wrote some text before a tool call: the OpenAI renderer made one message
per event, so the turn became a text message and a tool-call message in
a row, and llama.cpp answered 500. The renderer now folds consecutive
user or assistant messages into one, which is the OpenAI format for a
turn anyway. Checked against the live server: the captured failing
request no longer occurs, over six real runs.

LiteLLMClient gave up on any request after a fixed 120 s. A dense 27B
model writing a whole module in one turn takes longer, so the executor
escalated work that was going well. llm.request_timeout_s now sets it
for both clients, default 600.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

docs/Tuning-Guide.md

@@ -132,6 +132,21 @@ a local model can be held to less without affecting the frontier roles.
   executor. Frontier planning wants the large default. Leave this high, and
   set a local model's smaller ceiling as its `max_output_tokens` in LiteLLM.
 
+### `llm.request_timeout_s` — default 600
+
+How long one model request may take before it is abandoned. It applies to
+every request, on both routes.
+
+- **Too low:** a slow model's long answer is cut off, and the executor
+  escalates work that was going well. At 120 s, the old fixed value, a dense
+  27B model on the RTX 4090 timed out writing one module in one turn.
+- **Too high:** a hung connection holds the milestone for that long before
+  anything notices.
+- **Deciding:** this is a safety net, not a budget. `max_output_tokens` and
+  `max_turns` bound the work; set this above the slowest single answer you
+  expect, which is the model's output ceiling divided by its generation
+  speed.
+
 ### `tuning.executor.max_turns` — default 24
 
 How many model calls one milestone may make before the executor stops with