[codex] Extract chat turn phases

[roackb2]

Summary

Extracts M10 and M11 chat turn orchestration phases:

• adds turn-session.ts, turn-runtime.ts, and turn-tools.ts for ordinary turn session/runtime/tool preparation
• adds turn-memory-maintenance.ts for inline/background maintenance, trace appending, latest-turn event summary updates, and background failure trace events
• adds turn-persistence.ts for final turn persistence orchestration and final-compaction running seed updates
• keeps ordinary-turn.ts as high-level phase orchestration with lease cleanup still in the finally path
• documents the new chat modules in src/core/chat/README.md
• adds focused unit tests for runtime preparation and memory-maintenance trace update behavior

Why

This advances the domain-module refactor by shrinking the ordinary chat turn harness into named core-only phase modules while preserving public turn behavior.

Validation

• yarn test:unit src/__tests__/unit/core/chat-turn-runtime.test.ts
• yarn test:unit src/__tests__/unit/core/chat-turn-memory-maintenance.test.ts
• yarn vitest run src/__tests__/integration/memory/memory-integration.test.ts
• yarn vitest run src/__tests__/integration/chat/chat-runtime.test.ts
• yarn vitest run src/__tests__/integration/chat/session-submit.test.ts
• yarn vitest run src/__tests__/integration/core/agent-loop.test.ts
• yarn test:integration outside the sandbox
• yarn eslint
• yarn typecheck
• yarn build