chore: configure submodules to track main branch

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.

.gitmodules

@@ -13,6 +13,8 @@
 [submodule "mods/ztools"]
 	path = mods/ztools
 	url = https://git.kindred-systems.com/forbes/ztools.git
+	branch = main
 [submodule "mods/silo"]
 	path = mods/silo
 	url = https://git.kindred-systems.com/kindred/silo-mod.git
+	branch = main