fix: unblock CI collection, align ruff target, drop core-only quality_scale

- pyproject.toml: add pythonpath = ["."] so tests/ can import
  custom_components.openai_compatible (CI was red at collection without it)
- pyproject.toml: ruff target-version py313 -> py314, matching HA 2026.9's
  floor (already reflected in the CI workflow and requirements_test.txt)
- .gitignore: add .ruff_cache/
- remove custom_components/openai_compatible/quality_scale.yaml — a
  core-repo-only concept; the manifest no longer declares a quality scale,
  so the file asserted a tier this fork hasn't earned

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01739Seac5eMAfNwFeiwV5r4

.gitignore

@@ -1,3 +1,4 @@
 __pycache__/
 *.pyc
 .pytest_cache/
+.ruff_cache/