feat: add xAI Grok provider

[igmarin]

Why

ROADMAP item 2. The surrounding skill ecosystem already runs on Grok; SkillBench had no xAI client.

What

• Clients::Providers::Xai (OpenAI-compatible, same shape as Mistral/OpenRouter)
• Base URL https://api.x.ai, path /v1/chat/completions, default model grok-4
• skill-bench init --xai
• Env: SKILL_BENCH_XAI_API_KEY / XAI_API_KEY, SKILL_BENCH_XAI_MODEL / XAI_MODEL
• README, first-eval-guide, architecture

Does not add AWS Bedrock.

Proof

bundle exec ruby -Itest test/evaluator/clients/providers/xai_test.rb
# 7 runs, 0 failures

bundle exec ruby -Itest test/evaluator/config/env_overrides_test.rb
# 3 runs, 0 failures

skill-bench --help
# Providers include --xai

Summary by CodeRabbit

• New Features

  • Added xAI (Grok) as a supported provider.
  • Uses the OpenAI-compatible chat completions API with grok-4 as the default model.
  • Supports configuration through XAI_API_KEY or SKILL_BENCH_XAI_API_KEY, plus model overrides.
  • Added --xai initialization support.

• Documentation

  • Updated provider lists, setup guidance, configuration references, and architecture documentation.

• Tests

  • Added coverage for xAI requests, authentication, defaults, error handling, and environment-variable configuration.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Repository UI (base), Organization UI (inherited)

Review profile: CHILL

Plan: Advanced

Run ID: 09608336-231a-48ac-a3b1-1f672a0906b6

📥 Commits

Reviewing files that changed from the base of the PR and between 3f46789 and 0fe460f.

📒 Files selected for processing (13)

• CHANGELOG.md
• README.md
• docs/architecture.md
• docs/first-eval-guide.md
• lib/skill_bench/clients/README.md
• lib/skill_bench/clients/all.rb
• lib/skill_bench/clients/provider_schemas.rb
• lib/skill_bench/clients/providers/xai.rb
• lib/skill_bench/config/defaults.rb
• lib/skill_bench/config/env_overrides.rb
• test/evaluator/clients/providers/xai_test.rb
• test/evaluator/config/env_overrides_test.rb
• test/test_helper.rb

Included review availability: Your plan provides up to 1 included review per hour; 0 remain after this review.

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📝 Walkthrough

Walkthrough

The PR adds xAI as a provider. It defines the Grok client, default model, API endpoint, environment mappings, request tests, and documentation updates.

Changes

xAI provider integration

Layer / File(s)	Summary
Provider runtime and configuration lib/skill_bench/clients/providers/xai.rb, lib/skill_bench/clients/provider_schemas.rb, lib/skill_bench/config/..., lib/skill_bench/clients/all.rb	Registers the :xai provider, adds the Xai client, sets grok-4 as the default model, maps xAI environment variables, and loads the provider.
Provider behavior validation test/evaluator/clients/providers/xai_test.rb, test/evaluator/config/env_overrides_test.rb, test/test_helper.rb	Tests xAI requests, authentication, response extraction, missing-key errors, metadata, defaults, environment precedence, and environment cleanup.
Provider documentation README.md, CHANGELOG.md, docs/..., lib/skill_bench/clients/README.md	Documents xAI availability, configuration variables, initialization flags, API details, registry data, and provider inheritance.

Priority: ⬇️ Low

Estimated code review effort: 3 (Moderate) | ~25 minutes

Change: Feature

Sequence Diagram(s)

sequenceDiagram
  participant Caller
  participant Xai
  participant xAI_API as xAI API
  Caller->>Xai: submit chat messages and API key
  Xai->>xAI_API: POST /v1/chat/completions
  xAI_API-->>Xai: return completion content
  Xai-->>Caller: return result

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Merge Risk: ⚪ Minimal · up to 0fe46

The xAI provider configuration and documentation do not retain a supported actionable defect. The change is ready to merge after normal checks.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 23.08% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 13 functions across 8 files. (5 skipped: …	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely identifies the main change: adding the xAI Grok provider.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

Full details: Docstring Coverage

Explanation

Docstring coverage is 23.08% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 13 functions across 8 files. (5 skipped: 5 unsupported.)

• Fix all pre-merge checks with AI

✨ Finishing Touches 💡 1

📝 Generate docstrings 💡

• Create stacked PR
• Commit on current branch

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_{A rabbit found a Grok-shaped key,
And wired the paths from tree to API.
grok-4 hopped into the default seat,
Bearer tokens made the call complete.
Tests twitched their noses: all was neat.}

_{Comment @coderabbitai help to get the list of available commands.}