Add branch = main to mods/silo and mods/ztools in .gitmodules.
Add branch = main to silo-client in mods/silo/.gitmodules.
Enables 'git submodule update --remote' to auto-advance Kindred
submodules to latest main. Third-party deps (GSL, googletest,
AddonManager) remain pinned.
Also updates submodule pointers to latest main commits.
Syncing and update submodule should work, but if it doesn't then the following will reset it (+ remove all local changes):
```sh
git submodule deinit -f src/3rdParty/OndselSolver
git submodule update --init --recursive src/3rdParty/OndselSolver
```
