ci: auto-deploy docs to Cloudflare Pages

[ProKil]

Problem

PR #59 adds docs/team_harness_ablation_report.html but it was never auto-deployed — because CooperBench has no docs-deployment pipeline:

• The repo's original .github/workflows/docs.yaml (MkDocs + mike) was deleted in b61bc00 ("Updating linter"). It was unconfigured cookiecutter scaffolding anyway (referenced a placeholder src/your_package_name/__about__.py, needed a mkdocs.yml that never existed).
• GitHub Pages is still "enabled" but points at a gh-pages branch that doesn't exist, so the live site (cooperbench.github.io/CooperBench/) currently returns 404.

Fix

Add a Cloudflare Pages workflow modeled on cooperbench/CooperTrain's deploy-report.yml:

• Triggers on push to main and PRs touching docs/** (plus workflow_dispatch).
• Generates a docs/index.html landing page listing every docs/*.html report (title pulled from each file's <h1>, newest first, LATEST badge).
• Deploys the docs/ directory to the Cloudflare Pages project cooperbench-reports.
• PRs get a preview deployment + a comment with the preview URL.

The report is self-contained HTML (inline CSS, no runtime data fetches), so no static-site generator is needed.

⚠️ Required before it works: secrets

CooperBench has no Cloudflare secrets yet. The deploy step skips gracefully (green, with a warning) until both are set — same token/account CooperTrain uses:

gh secret set CLOUDFLARE_API_TOKEN  --repo cooperbench/CooperBench
gh secret set CLOUDFLARE_ACCOUNT_ID --repo cooperbench/CooperBench

(Or promote them to org-level secrets so both repos share them.)

Notes / decisions

• Separate Pages project (cooperbench-reports), not CooperTrain's cooperbench-train-reports: each Cloudflare Pages deploy replaces the whole project's content, so reusing CooperTrain's project would clobber its reports. This keeps both on the same Cloudflare account (shared secrets) but on distinct sites. Rename the --project-name if you'd rather merge them.
• Deploys the whole docs/ dir, so docs/team_harness_ablation_report.html lands at cooperbench-reports.pages.dev/team_harness_ablation_report. PR docs: team-harness ablation report (flash, codex/gpt-5.5) #59's report deploys automatically once both this and docs: team-harness ablation report (flash, codex/gpt-5.5) #59 are on main.

🤖 Generated with Claude Code