ci(python-tests): diff CLI output against the PR base branch

The agent-facing CLI output differential defaulted to origin/main, so a PR
whose base branch is not main (e.g. frontend-control-plane-im-prototype-rfc)
failed whenever main moved a shared baseline such as the heartbeat-prompt
portfolio. Point LOOPX_CLI_OUTPUT_BASE_REF at the pull request base branch,
falling back to origin/main for main pushes.

Author: xiaods

.github/workflows/python-tests.yml

@@ -55,6 +55,8 @@ jobs:
         run: python -m mypy
 
       - name: Qualify agent-facing CLI output
+        env:
+          LOOPX_CLI_OUTPUT_BASE_REF: origin/${{ github.event.pull_request.base.ref || 'main' }}
         run: python examples/control_plane/cli-output-budget-regression-smoke.py
 
       - name: Run fast tests