fix(git): select USC Gitea login for PR merges (#516)

packages/mosaic/framework/tools/git/detect-platform.sh

@@ -91,6 +91,31 @@ get_remote_host() {
     return 1
 }
 
+# Resolve the tea login name for the given Gitea host.
+# Priority: explicit caller override → known Mosaic host mapping → no forced login.
+get_gitea_login() {
+    local host="$1"
+
+    if [[ -n "${GITEA_LOGIN:-}" ]]; then
+        echo "$GITEA_LOGIN"
+        return 0
+    fi
+
+    case "$host" in
+        git.mosaicstack.dev)
+            echo "mosaicstack"
+            return 0
+            ;;
+        git.uscllc.com)
+            echo "usc"
+            return 0
+            ;;
+        *)
+            return 1
+            ;;
+    esac
+}
+
 # Resolve a Gitea API token for the given host.
 # Priority: Mosaic credential loader → GITEA_TOKEN env → ~/.git-credentials
 get_gitea_token() {
@@ -104,6 +129,10 @@ get_gitea_token() {
         local token
         token=$(
             source "$cred_loader"
+            # load_credentials preserves pre-existing env vars by design. Clear
+            # Gitea env in this subshell so host-specific credential lookup wins
+            # over an ambient token for a different Gitea instance.
+            unset GITEA_TOKEN GITEA_URL
             case "$host" in
                 git.mosaicstack.dev) load_credentials gitea-mosaicstack 2>/dev/null ;;
                 git.uscllc.com)      load_credentials gitea-usc 2>/dev/null ;;