Add Firecrawl web research eval docs

[rforgeon]

Summary

Adds a small harness-neutral eval set for Firecrawl web research workflows while keeping Codex-specific plugin metadata in the dedicated firecrawl/firecrawl-codex-plugin repo.

What changed

• Add human-review eval cases for search, scrape, and site-map workflows
• Add a rubric for tool selection, source handling, output usefulness, and safety
• Point Codex users to the separate Firecrawl Codex plugin repository
• Document an optional Telvine publishing path with metadata-only telemetry guidance

Validation

jq . .claude-plugin/plugin.json .claude-plugin/marketplace.json
git diff --check

Note: this PR intentionally does not add .codex-plugin or .agents/plugins metadata because Firecrawl already maintains that in firecrawl/firecrawl-codex-plugin.