One round for a chat, as many as it takes for an agent

Two different jobs were sharing one number. A plain conversation asking a
question is one round of looking things up and then an answer; the rounds after
that were a small model that had decided searching was the answer searching
until the context ran out, at a full request each. MAX_ROUNDS is 1 now. Several
tools can still be called within that round, which is the thing worth telling
the model.

The trade is real and worth naming: a plain chat can no longer search and then
read one of the results, because reading is a second round. That is what an
agent chat is for.

An agent chat is sized by Limits instead, where steps is now a runaway backstop
and not a working budget. It was 40 and it was reached -- a step count low
enough to be the thing that ends a reply is a count that ends it halfway. What
bounds one now is the wall clock and a new completion-token ceiling, with zero
meaning no ceiling, the same convention index_chars already uses.

That ceiling would have been decorative. generation.completion_tokens is only
populated when the endpoint sends a usage block, and llama.cpp, Ollama and
friends never do; the fallback estimate is computed once, in _run's finally,
long after the loop that needs it. So _written takes the larger of reported and
estimated, and there is a test that runs the whole thing against a stream
reporting no usage at all. A limit that works on OpenAI and silently does
nothing everywhere else is the worst kind: one that looks configured.

core.rounds could not stay one fragment. "You get at most N rounds" is not the
same sentence with a different number in it -- a model told it has a budget
rations it and stops early to report progress, which is exactly the behaviour
that strands a long piece of work. So it splits: core.rounds keeps the
one-round case and gates on a new round_budget variable that _agent_values
blanks, and core.keep_working says the other thing to an agent chat.

A queued message during a one-round reply is now never taken mid-reply -- there
is no work under way to steer -- and falls through to _drain, which gives it a
reply of its own. No code change went with that; it falls out of the guard, and
there is a test so that "it happens to work" and "it is meant to work" stop
looking the same.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/lembas/web/templates/admin/agents.html

@@ -127,15 +127,22 @@
   <section class="card">
     <h2 class="card__title">What one reply may spend</h2>
     <p class="field__hint">
-      Three separate bounds, because they fail differently: steps stop a loop,
-      the clock stops one slow command eating an afternoon, and output stops a
-      model filling its own context with build logs and having no room to answer.
+      Four separate bounds, because they fail differently: the clock stops one
+      slow command eating an afternoon, tool output stops a model filling its own
+      context with build logs and having no room to answer, written tokens stop
+      one that keeps going, and the step count is a backstop against a runaway.
     </p>
 
     <div class="field">
-      <label class="field__label" for="max_steps">Most rounds of tool calls</label>
-      <input class="input" id="max_steps" name="max_steps"
-             value="{{ values.max_steps }}" inputmode="numeric">
+      <label class="field__label" for="max_completion_tokens">
+        Most a reply may write
+      </label>
+      <input class="input" id="max_completion_tokens" name="max_completion_tokens"
+             value="{{ values.max_completion_tokens }}" inputmode="numeric">
+      <p class="field__hint">
+        In tokens, across every round of one reply. This is the bound that
+        normally ends a long piece of work. Zero means no ceiling.
+      </p>
     </div>
     <div class="field">
       <label class="field__label" for="max_wall_seconds">Longest a reply may take</label>
@@ -148,6 +155,16 @@
       <input class="input" id="max_total_output_bytes" name="max_total_output_bytes"
              value="{{ values.max_total_output_bytes }}" inputmode="numeric">
     </div>
+    <div class="field">
+      <label class="field__label" for="max_steps">Most rounds of tool calls</label>
+      <input class="input" id="max_steps" name="max_steps"
+             value="{{ values.max_steps }}" inputmode="numeric">
+      <p class="field__hint">
+        A backstop, not a working budget. An agent reply is meant to run until
+        the task is done, so a number low enough to be what stops it is a number
+        that stops it halfway. Use the token ceiling above for a real limit.
+      </p>
+    </div>
   </section>
 
   <section class="card">