fix(ci-queue-wait): treat absent branch (404) as queue-clear (#872)

Closes #872

Mos (id-11) Gate-16 merge: independent review APPROVE @23cbdaf8, author jason.woltje(id2) != approver Mos(id11), CI green wp1974.

Co-authored-by: jason.woltje <jason@diversecanvas.com>
Co-committed-by: jason.woltje <jason@diversecanvas.com>

packages/mosaic/framework/tools/git/ci-queue-wait.sh

@@ -137,7 +137,21 @@ gitea_get_branch_head_sha() {
     local branch="$3"
     local token="$4"
     local url="https://${host}/api/v1/repos/${repo}/branches/${branch}"
-    curl -fsSL -H "User-Agent: curl/8" -H "Authorization: token ${token}" "$url" | python3 -c '
+    # Capture HTTP status so an absent branch (404) is distinguished from an API
+    # error. A not-yet-pushed feature branch has no in-flight pipeline, so the
+    # pre-push queue guard must treat 404 as "queue clear", not crash.
+    local resp code body
+    resp=$(curl -sS -H "User-Agent: curl/8" -H "Authorization: token ${token}" -w $'\n%{http_code}' "$url")
+    code="${resp##*$'\n'}"
+    body="${resp%$'\n'*}"
+    if [[ "$code" == "404" ]]; then
+        echo "__BRANCH_ABSENT__"
+        return 0
+    fi
+    if [[ "$code" != "200" ]]; then
+        return 1
+    fi
+    printf '%s' "$body" | python3 -c '
 import json, sys
 data = json.load(sys.stdin)
 commit = data.get("commit") or {}
@@ -219,6 +233,10 @@ elif [[ "$PLATFORM" == "gitea" ]]; then
         exit 1
     }
     HEAD_SHA=$(gitea_get_branch_head_sha "$HOST" "$OWNER/$REPO" "$BRANCH" "$TOKEN")
+    if [[ "$HEAD_SHA" == "__BRANCH_ABSENT__" ]]; then
+        echo "[ci-queue-wait] branch ${BRANCH} not yet on remote — no in-flight pipeline; queue clear."
+        exit 0
+    fi
     if [[ -z "$HEAD_SHA" ]]; then
         echo "Error: Could not resolve ${BRANCH} head SHA." >&2
         exit 1