docs(agents): add AGENTS.md documentation for prometheus, hephaestus, sisyphus variants, and builtin-skills

- src/agents/prometheus/: Strategic planner documentation
- src/agents/hephaestus/: GPT-5.4 autonomous worker documentation
- src/agents/sisyphus/: Model-specific orchestrator variants documentation
- src/features/builtin-skills/: 8 built-in skills catalog

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