docs: introduce evals/ as the canonical skill-behavior eval harness

- docs/testing.md split into Plugin tests + Skill behavior evals.
  Plugin tests section enumerates the bash tests that survive
  (kept by drill-coverage analysis or as describe-skill tests).
- CLAUDE.md adds Eval harness section pointing at evals/.
- README.md Contributing section mentions evals/ alongside tests/.
- .gitignore adds evals/{results,.venv,.env} as belt-and-suspenders
  (evals/.gitignore covers these locally; root-level entries help
  tooling that does not recurse into nested ignore files).

.gitignore

@@ -5,3 +5,9 @@
 node_modules/
 inspo
 triage/
+
+# Eval harness — drill ships its own gitignore at evals/.gitignore;
+# these are belt-and-suspenders entries for tools that don't recurse.
+evals/results/
+evals/.venv/
+evals/.env