[codex] Complete M11.5 chat turn cleanup

[roackb2]

Summary

• finish M11.5-E by extracting ordinary turn context preparation and run-loop execution phases
• move preflight compaction-running seed persistence into the preflight module
• remove the core chat turn path's value import from the package root and update integration tests to spy on the runtime module directly
• update chat README for the settled turn modules

Verification

• yarn vitest run src/tests/unit/core/chat-turn-runtime.test.ts src/tests/unit/core/chat-turn-memory-maintenance.test.ts
• yarn vitest run src/tests/integration/chat/chat-runtime.test.ts src/tests/integration/chat/session-submit.test.ts
• yarn vitest run src/tests/integration/tui/ask-cli.test.ts -t "runs a stateless ask|creates a new session|continues an existing session|preflight compacts"
• yarn vitest run src/tests/integration/memory/memory-integration.test.ts
• yarn test:unit
• yarn eslint
• yarn typecheck
• yarn build
• git diff --check

Note

Full yarn test:integration was attempted. It still fails in this sandbox on existing temp git identity failures in workspace-diff tests and localhost 127.0.0.1 bind EPERM in daemon ask-cli tests; the non-daemon ask-cli coverage touched by this PR passes.