docs: add LLM-as-Judge examples and repo guidance

[dpaluy]

Summary

• Update the README quick-start judge model to gpt-5.5 and add a pointer to the standalone examples guide.
• Add repository-local AGENTS.md guidance for future agent work in this gem.
• Add standalone LLM-as-Judge examples for RAG scoring, batch pass/fail output, model comparison, custom metrics, and Minitest assertions.
• Bump the gem version to 0.1.2, add changelog notes, and exclude examples from packaged gem files.

Why

Users asked for practical LLM-as-Judge examples that are easy to demo and adapt. This PR keeps those examples separate from the main README while still linking to them, and it includes the repo-local guidance and ignore rule needed for future agent work.

Changes

docs(examples): make LLM-as-judge demos self-contained

• What: Added live Ruby examples under examples/ plus a detailed examples README covering setup, run commands, expected output, and interpretation.
• What: Added batch output that prints per-sample PASS/FAIL decisions and metric-score breakdowns.
• What: Bumped RubricLLM::VERSION to 0.1.2 and added CHANGELOG.md release notes.
• What: Updated rubric_llm.gemspec to exclude examples/ from packaged gem files.
• Why: Keep adoption examples separate from the root README while preserving a lean runtime gem package.

docs: add repository guidance and examples pointer

• What: Added AGENTS.md with repo structure, test, style, and security guidance.
• What: Updated the root README quick-start model to gpt-5.5 and linked to examples/README.md.
• Why: Keep local workflow state out of git and make the examples discoverable from the main README.

Testing

• bundle exec rake
• package check confirmed version=0.1.2, examples_in_package=[], and agents_in_package=[]
• gem build rubric_llm.gemspec --output /tmp/rubric_llm-0.1.2.gem
• git diff --check
• Live provider example scripts were not run because they require OpenAI API calls.

Risk & Rollout

• Risk: Low. Most changes are documentation/examples, and examples plus AGENTS.md are excluded from packaged gem files.
• Rollback: Revert the PR commits to remove examples, repo guidance, README edits, and release/package metadata.
• Monitoring: Confirm release packaging still excludes examples before publishing 0.1.2.