This looks fine to me, but I also have no idea what I'm doing. I'd appreciate someone with more arc experience letting me know if "history.immutable" : false being missing from the included-in-repo arcconfig will be a burden on those of us that have history mutation enabled...

@durham @quark

This looks good to me too.

Personally I would vote for setting history.immutable to false too. This mostly affects the behavior of the arc diff and arc amend commands. This probably won't matter at all for people that submit diffs using the phabsend extension instead of arc diff.
When set to false, arc diff will rewrite your local commit message to include the differential revision ID when submitting a diff. arc amend will update your commit message to add a Reviewed By line once your diff has been accepted.
Personally I find it useful to have this information in my local commit messages. (The phabdiff and phabstatus extensions in the rFBHGEXT repository can also report some useful information if they find this field present in your commit message.)

The only downside of leaving history.immutable as true is that you don't get this information in your commit message when running arc diff / arc amend; it shouldn't impact any other behavior.
One arguable downside of setting history.immutable to false is that your local commit IDs do change after running arc diff or arc amend if it needed to update your commit message.