CI workflow to run evals and update the leaderboard (#43)

Lets the leaderboard show real numbers without a local key: the new
"Update Skill Leaderboard" workflow (workflow_dispatch) runs the eval harness
with the ANTHROPIC_API_KEY secret, commits evals/results.json, and the Pages
deploy re-renders the public leaderboard with real data.

- .github/workflows/eval-leaderboard.yml: manual trigger, contents: write,
  runs run-evals.mjs + build-leaderboard.mjs, commits results.json.
- deploy-playground.yml: also trigger on evals/results.json (and the build
  scripts) so the committed results refresh the live page.
- evals/README + CHANGELOG document the CI route.


Claude-Session: https://claude.ai/code/session_016JWn5jRD5tcEFKrubjQ6Px

Co-authored-by: Claude <noreply@anthropic.com>

.github/workflows/deploy-playground.yml

@@ -10,6 +10,10 @@ on:
     paths:
       - 'skills/**'
       - 'web/**'
+      - 'evals/results.json'
+      - 'skill-tiers.json'
+      - 'scripts/build-docs.mjs'
+      - 'scripts/build-leaderboard.mjs'
       - '.github/workflows/deploy-playground.yml'
   workflow_dispatch: