refactor: eliminate routing layer, align with industry best practices
Phase 1 of architecture optimization (U1/U2/U4/U8):
- U1: Rename SimpleRouter to RequestPreprocessor, route() to preprocess()
Eliminates misleading routing concept; LLM decides autonomously
in REACT agent loop (matches Codex/Claude Code/Trae pattern)
- U2: Delete CostAwareRouter, HeuristicClassifier, SemanticRouter
(~700 lines removed). skill_routing.py: 1688 to 220 lines
- U4: PlanExecEngine defaults to ReActStepExecutor, delete _LLMStepExecutor
(pure LLM calls without tools = no execution capability)
- U8: ReActEngine defaults to ContextCompressor(keep_recent=10)
Supersedes plans 2026-06-15-002/003/004.
New plan: 2026-06-16-006-refactor-architecture-optimization-evolution-plan.md
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