fix(agents): AgentInterface declared a coroutine where every caller wants a generator

`generate_response` was `async def` with a `pass` body and no `yield`.
An async function that never yields is a coroutine, so the declared type
was Coroutine[..., AsyncGenerator[OutputItem, None]] — something a
caller must await before it can be iterated.

Nobody awaits it. Both implementations contain yields (TatlockAgent 5,
LoremTesterAgent 3), which makes them async generators directly, and
both call sites do `async for item in agent.generate_response(...)`.
The abstract method's own docstring says "Yields:" and its own example
iterates the call without awaiting. Implementations, consumers and prose
all agreed; only the declaration dissented.

Removing one word fixes it, and it is the declaration that was wrong
rather than the four places reporting it.

WHY NOTHING CAUGHT THIS. The abstract body is `pass` and nothing calls
super().generate_response — verified across the tree — so the wrong
declaration has no runtime consequence and cannot fail a test. It was
invisible by construction, and it presented as four unrelated errors in
four files (two override, two attr-defined), none of which named the
cause. Anyone fixing them where they appeared would have annotated the
implementations to match the interface and made the real defect
permanent.

tests/agents/test_agent_interface.py covers it going forward. The
load-bearing case is not "the interface is X" or "the implementation is
Y" separately — both could drift together and still pass — but that the
two AGREE about what kind of callable this is.

Mutation-checked: restoring `async` fails 3 of the 5 new tests, the two
that still pass being the implementation checks, which are correctly
unaffected. Anchor asserted unique before the mutation was written, and
the fix asserted back into place afterwards.

95 errors -> 71 across this branch; this commit accounts for 4 of them.
Suite 662 passed, 1 failed — that failure is the known LLM-nondeterministic
calculator test, which passed on the previous run of this same branch and
failed on this one, which is the clearest available evidence that it is
unrelated to any of this work.

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

src/agents/base.py

@@ -36,7 +36,7 @@ class AgentInterface(ABC):
     """
 
     @abstractmethod
-    async def generate_response(
+    def generate_response(
         self,
         messages: list[dict],
         reasoning: dict | None = None,