default the ollama host

[codefromthecrypt]

If you choose the ollama provider, but don't give an ENV for the host, you'll receive an error leading to the lookup line like this:

../src/exchange/providers/ollama.py:45: in from_env
    url = os.environ["OLLAMA_HOST"]

I would suggest we default this, like tests do instead, to "http://localhost:11434/". Even if some tools exclusively require passing OLLAMA_HOST, I think we can justify the implicit default due the user already being explicit with the ollama provider config.

If they want to modify the host, they can still via env or any other mechanism we support in the future.

p.s. I'm happy to change this in a PR!

[codefromthecrypt]

will be signaling based on OLLAMA_HOST set in #23, so closing this unless someone else cares, in which case I'll re-open