[codex] Complete chat turn quality gate

[roackb2]

Summary

Completes the M11.6 chat-turn architecture-quality gate before moving to toolkits.

• Delete the wrapper-only src/core/chat/turn-tools.ts module.
• Keep ordinary-turn default tool setup directly visible in turn-context.ts.
• Move prepared preflight session persistence into session-turn-preflight.ts.
• Move completed-turn session saving into turn-persistence.ts.
• Remove src/core/chat/* imports from the public package root and add an import-boundary test to prevent recurrence.
• Add behavior tests for approval bridging, lease cleanup on run-loop failure, preflight persistence, and final turn persistence.
• Update src/core/chat/README.md to distinguish stable core entry points from internal turn services.

Why

M11.6 is meant to be a quality gate, not a file shuffle. The chat turn path now leaves ordinary-turn.ts as orchestration over behavior-owning services: context preparation, preflight, run loop, memory maintenance, persistence, and lease cleanup. Durable side effects now live with the lifecycle phase that owns them.

Validation

• yarn test:unit src/__tests__/unit/core/chat-turn-runtime.test.ts src/__tests__/unit/core/chat-turn-persistence.test.ts src/__tests__/unit/core/import-boundaries.test.ts
• yarn vitest run src/__tests__/integration/chat/chat-runtime.test.ts
• yarn vitest run src/__tests__/integration/chat/session-submit.test.ts src/__tests__/integration/memory/memory-integration.test.ts
• yarn typecheck
• yarn eslint