MAINT: add nightly upstream-dev CI job

[mgrover1]

Every xradar/xarray release has historically broken the accessor after the fact (#1848, #1700, #1677–#1682); we find out when the updates ship. This adds .github/workflows/upstream-ci.yml, modeled on xarray's upstream-dev CI and MetPy's nightly checks:

• Daily cron (guarded to ARM-DOE/pyart), workflow_dispatch, and label-gated PR runs (run-upstream-ci).
• Installs xradar and xarray from git main plus nightly numpy/scipy/pandas wheels (scientific-python-nightly-wheels), on top of the regular CI environment.
• On scheduled failure, auto-opens/updates an issue via scientific-python/issue-from-pytest-log-action — with a fallback that still files an issue when the environment breaks before pytest runs (the most likely upstream-breakage mode).
• One line in CONTRIBUTING.rst pointing contributors at the workflow.

Post-merge checklist: trigger once via workflow_dispatch to validate on real runners; create the CI and upstream labels if absent.

Validation: actionlint clean; both failure paths reviewed.