Context

Part of kcenon/common_system#685 (Wave 2, P1).

Acceptance Criteria

• dedup ARCHITECTURE/BENCHMARKS/CONTRIBUTING/FAQ/CHANGELOG/MIGRATION root vs subdir to single SSOT
• unify samples/ and examples/ (topic overlap basic_usage/transaction/async) with clear role boundary
• refresh or mark-superseded ADR-002 (connection-pool) vs current pooling state
• README.kr.md raised to >= 70% of README.md

Derived from the 2026-05-30 multi-agent cross-system level-matching analysis (workflow wf_0a968699-e07). Reference systems: documentation=common_system, code-structure=monitoring_system, tests/examples=container_system.