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>

tests/test_harness.py

@@ -357,3 +357,26 @@ def test_a_plain_chat_is_told_nothing_about_files(db, owner):
     db.commit()
 
     assert "Files in" not in harness.compose(db, owner, _tools("web_search"), chat=chat)
+
+
+# --- One round, or as many as it takes ----------------------------------------
+def test_a_plain_chat_is_told_it_has_one_round(db, owner):
+    """And is told to ask for everything at once, which is the advice that
+    matters when there is only one."""
+    text = harness.compose(db, owner, _tools("web_search"))
+
+    assert "one round of tool calls" in text
+    assert "Keep working until the task is actually done" not in text
+
+
+def test_an_agent_chat_is_told_to_keep_going_instead(db, owner):
+    """The two cannot be one fragment with a number in it. A model told it has
+    a budget rations it; the step count is a runaway backstop, and rationing
+    against it is exactly the behaviour that stops a long piece of work
+    halfway."""
+    chat, _profile = _agent_chat(db, owner)
+
+    text = harness.compose(db, owner, _agent_tools(db), chat=chat)
+
+    assert "Keep working until the task is actually done" in text
+    assert "one round of tool calls" not in text