fix: install pytest for the queue-filing gate

First live run (#50) filed the paper cleanly but the gate died on
'No module named pytest' — the bare runner doesn't ship it. Add
setup-python + pip install pytest before the Claude step so both the
in-sandbox check and the deterministic gate can run the suite.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01K21CJX9TAgULDo1fFJtonc

Author: 2 people

.github/workflows/queue_filing.yml

@@ -63,6 +63,15 @@ jobs:
 
       - uses: actions/checkout@v4
 
+      - uses: actions/setup-python@v5
+        with:
+          python-version: "3.12"
+
+      # The gate below (and Claude's own in-sandbox check) runs pytest; the
+      # bare runner doesn't ship it — its absence failed the first live run.
+      - name: Install the test runner
+        run: pip install pytest
+
       - name: Stash the request for Claude
         env:
           # env, never inline interpolation — issue text is untrusted input.