Refine PR template, test markers, and add auto-labeler

[jmsexton03]

Summary

Updates the PR template to be more structured and DRY, standardizes solver-related test markers and naming for -k filtering, and adds a GitHub labeler workflow for automatic labels.

Related or overlapping functionality / DRY guidance

• Related to tests/README.md and marker definitions in pyproject.toml.
• Aligns with test gating logic in tests/conftest.py.
• Supports solver-specific test selection via -k pelec|erf|amrex|warpx.

Impact checklist

• fixes a bug or incorrect behavior
• adds new capabilities
• changes answers in the test suite to more than roundoff level
• likely affects downstream users or results
• includes docs updates (code/docs), if appropriate
• none of the above

Tests run (CI runs: pytest tests/unit, pytest tests/quality)

• tests/unit: pytest tests/unit
• tests/quality: pytest tests/quality
• tests/integration (if applicable): pytest tests/integration
• other (list): pytest tests/e2e -m e2e -k pelec -q, pytest tests/integration/test_input_writer_pipeline.py -q
• Output/summary:
  • pytest tests/quality passed (1 skipped: marker doc check).
  • pytest tests/e2e -m e2e -k pelec -q skipped (local requirements missing).
  • pytest tests/integration/test_input_writer_pipeline.py -q skipped (schema missing).

Tests not run in CI (required if any)

• CI only runs tests/unit and tests/quality via micromamba; list anything else here.
• None
• tests/integration
• other (list): tests/e2e
• Reason for skip: local repos/schemas/indices not available; e2e is opt-in.
• Risk/mitigation: No functional code changes; markers/tests are documented and gating is explicit.

Notes (optional)

• Manual output / logs (short):
• Known limitations: E2E/integration tests require local repos/schemas/indices.

Labels (optional)

• I will check auto-labels after submitting.
• Labels are okay if slightly off.