v4.2.1: Faster agent review, repo-aware context, and benchmark guards

[duriantaco]

Changed

• skylos agent scan now defaults to the fast review path. Slow dead-code verification is opt-in via --verify-dead-code
• Agent review is more repo-aware, with better file selection and context for quality, security, and debt-style issues
• Added agent benchmarks and Codex comparison runs with token reporting

Fixed

• Agent scans now fail cleanly on missing API keys instead of crashing
• Review output is clearer when dead-code verification is still running
• LLM provider and runtime settings now propagate correctly through the agent path

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This discussion was created from the release v4.2.1: Faster agent review, repo-aware context, and benchmark guards.