fix(tools/git): ci-queue-wait.sh treats 404 branch-absent as queue-clear

gitea_get_branch_head_sha() looked up a branch's head SHA via
`curl -fsSL .../branches/<branch>`. For a branch not yet pushed to the
remote, Gitea returns 404, curl -f fails, stdin is empty, and the
downstream `json.load` raises JSONDecodeError -- crashing the guard.
Since ci-queue-wait.sh is a mandatory pre-push check, every new feature
branch's first push was blocked.

Capture the HTTP status via `curl -sS -w '\n%{http_code}'`:
- 404  -> echo __BRANCH_ABSENT__, return 0 (no in-flight pipeline on a
          branch that doesn't exist yet -- queue is clear).
- non-200 -> return 1 (still fail-closed on a genuine API error).
- 200  -> parse the JSON as before (existing-branch behavior unchanged).

The caller checks for the __BRANCH_ABSENT__ sentinel and exits 0 with a
"branch not yet on remote -- queue clear" message.

ci-queue-wait.ps1 had the same bug (Invoke-RestMethod throws on any
non-2xx, including 404, and the catch block exited 1) -- ported the
equivalent fix there via the caught exception's HTTP status code.

Adds a red-first regression harness (test-ci-queue-wait-branch-absent.sh)
covering: 404 branch-absent -> queue clear; 200 existing branch with a
terminal CI state -> unchanged behavior; 500 genuine API error -> still
fail-closed. Verified failing against the unpatched script (crashes with
JSONDecodeError) and passing after the fix. Wired into
package.json's test:framework-shell alongside the existing git-wrapper
regression tests.

Upstreams Mos host-local tooling-patch kit (2026-07-23), Patch 3.

packages/mosaic/framework/tools/git/ci-queue-wait.ps1

@@ -185,6 +185,16 @@ switch ($platform) {
             $headSha = ($branchPayload.commit.id | Out-String).Trim()
         }
         catch {
+            # 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.
+            $statusCode = $null
+            if ($_.Exception.Response) {
+                $statusCode = [int]$_.Exception.Response.StatusCode
+            }
+            if ($statusCode -eq 404) {
+                Write-Host "[ci-queue-wait] branch $Branch not yet on remote — no in-flight pipeline; queue clear."
+                exit 0
+            }
             Write-Error "Could not resolve $Branch head SHA from Gitea API."
             exit 1
         }