[codex] Remove thin chat turn execution wrapper

[roackb2]

Summary

• delete src/core/chat/turn-execution.ts because it only wrapped runAgentLoop option assembly for one caller
• put the runAgentLoop call back in src/core/chat/ordinary-turn.ts so the turn lifecycle remains visible
• keep only the host approval bridge as a private helper because it translates the chat host approval port into the runtime callback shape
• remove the stale README entry for the deleted wrapper

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 eslint
• yarn typecheck
• git diff --check