This PR is motivated by internal feedback in which the notification that
we show inviting to resolve merging conflicts with an agent also pops up
if the agent itself ran `git merge`. In this case, the notification is
unnecessary noise. So, what I'm doing here is simply _not_ showing it if
there's a running agent.

I want to note that this change is accepting a trade-off here, in which
there could be cases that even if an agent is running, the notification
can still be useful. There could be other ways to identify whether the
agent is running `git merge`, but they all felt a bit too complex for
the moment. And given this is reasonably an edge case, I'm favoring a
simple approach for now.

Release Notes:

- N/A

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Co-authored-by: Lukas Wirth <lukas@zed.dev>