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/package.json

@@ -25,7 +25,7 @@
     "lint": "eslint src",
     "typecheck": "tsc --noEmit",
     "test": "vitest run --passWithNoTests && pnpm run test:framework-shell",
-    "test:framework-shell": "python3 src/lease-broker/daemon_deadline_unittest.py && python3 src/lease-broker/normative_fragments_unittest.py && python3 src/lease-broker/receipt_challenge_unittest.py && python3 src/lease-broker/context_recovery_unittest.py && python3 src/lease-broker/recovery_runtime_unittest.py && python3 src/lease-broker/recovery_b1_adversarial_unittest.py && python3 src/lease-broker/framework_skill_portability_unittest.py && python3 src/mutator-gate/runtime_tools_unittest.py && python3 src/mutator-gate/runtime_launch_guard_unittest.py && python3 framework/tools/lease-broker/check-runtime-launches.py --root ../.. && bash framework/tools/codex/test-pr-diff-context.sh && bash framework/tools/qa/test-deps-preflight.sh && bash framework/tools/git/test-pr-review-gitea-comment.sh"
+    "test:framework-shell": "python3 src/lease-broker/daemon_deadline_unittest.py && python3 src/lease-broker/normative_fragments_unittest.py && python3 src/lease-broker/receipt_challenge_unittest.py && python3 src/lease-broker/context_recovery_unittest.py && python3 src/lease-broker/recovery_runtime_unittest.py && python3 src/lease-broker/recovery_b1_adversarial_unittest.py && python3 src/lease-broker/framework_skill_portability_unittest.py && python3 src/mutator-gate/runtime_tools_unittest.py && python3 src/mutator-gate/runtime_launch_guard_unittest.py && python3 framework/tools/lease-broker/check-runtime-launches.py --root ../.. && bash framework/tools/codex/test-pr-diff-context.sh && bash framework/tools/qa/test-deps-preflight.sh && bash framework/tools/git/test-pr-review-gitea-comment.sh && bash framework/tools/git/test-ci-queue-wait-branch-absent.sh"
   },
   "dependencies": {
     "@mosaicstack/brain": "workspace:*",