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_agent_profiles.py

@@ -391,9 +391,10 @@ def test_the_numbers_are_clamped(client: TestClient, db, registered):
             "default_timeout": "0",
             "max_timeout": "99999",
             "max_output_bytes": "1",
-            "max_steps": "9999",
+            "max_steps": "99999",
             "max_wall_seconds": "1",
             "max_total_output_bytes": "1",
+            "max_completion_tokens": "0",
             "approval_timeout": "0",
             "allow_default": "",
             "deny_default": "",
@@ -403,8 +404,11 @@ def test_the_numbers_are_clamped(client: TestClient, db, registered):
     values = settings_store.agents(db)
     assert values["default_timeout"] == 1
     assert values["max_timeout"] == 3600
-    assert values["max_steps"] == 200
+    assert values["max_steps"] == 1000
     assert values["approval_timeout"] == 60, "a zero would park a task forever"
+    # Not clamped up to a minimum: zero is how "no ceiling on what a reply may
+    # write" is said, exactly as it is for index_chars.
+    assert values["max_completion_tokens"] == 0
 
 
 def test_an_unticked_checkbox_turns_it_off(client: TestClient, db, registered):