fix(tests): pin init.defaultBranch in test-mkrep to silence git's hint

Every mkrep call in this suite runs a bare `git init`. On a runner with
no init.defaultBranch configured, git prints its "Using 'master' as the
name for the initial branch" advice block on each one (20 occurrences in
CI's test-job log, all from this file). test-agents-vault.fish already
pins the same setting via GIT_CONFIG_COUNT/KEY/VALUE for the identical
reason; apply the same fix here rather than in mkrep.fish itself, so a
real user's own git config still wins in normal use.

tests/test-mkrep.fish

@@ -35,6 +35,15 @@ set -gx GITEA_HOST ''
 set -gx GITLAB_URL ''
 set -gx GITLAB_HOST ''
 
+# init.defaultBranch is pinned for the same reason test-agents-vault.fish
+# pins it: a developer's global config says "main", a runner with no config
+# falls back to "master" and prints git's advice hint on every `git init`
+# this file makes. Pinning it here (not in mkrep.fish itself) keeps a real
+# user's own global git config the one that wins in normal use.
+set -gx GIT_CONFIG_COUNT 1
+set -gx GIT_CONFIG_KEY_0 init.defaultBranch
+set -gx GIT_CONFIG_VALUE_0 main
+
 function _mkrep_sandbox
     set -l tmp (mktemp -d)
     # Resolve symlinks (e.g. macOS/NixOS /tmp) so path comparisons against