feat(FN-1443): create dedicated soul.md files for all New Agent presets
- Add soul.md files for 20 agent presets (backend-engineer, ceo, cfo, ci-engineer, cmo, cto, data-engineer, designer, devops-engineer, engineer, frontend-engineer, fullstack-engineer, marketing-manager, ml-engineer, product-manager, qa-engineer, reviewer, security-engineer, technical-writer, triage)
- Create agent-presets/index.ts module exporting all preset definitions
- Refactor NewAgentDialog to use centralized agent-presets module
- Add comprehensive tests for agent presets validation
- Update docs/agents.md with agent preset documentation
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