Sandboxes: compatibility with Github Sync

[josephjclark]

GitHub Sync needs a few tweaks to be compatible with Sandboxes:

• When merging, make a pre-merge sync on the target (main) - with a message like Pre-commit before merging sandbox "staging"
• When merging, make a post-merge sync on the target (main) - with a message like Merged changes from sandbox "staging"

We also need to run a manual test that a sandbox can itself be configured to merge. This should be configurable through settings but NOT defaulted.

Note that this is not to be confused with v2 github sync with the new CLI: this is just adding feature parity to the existing sync