[codex] Implement prompt message history CLI

[simonw]

Summary

This implements the documentation-driven llm prompt message-history workflow introduced on the branch:

• adds --messages to load serialized Message JSON from an inline string, file path, or - for stdin
• adds repeatable -M/--message ROLE TEXT entries for text-only system, user, and assistant messages
• appends any regular prompt input as a final user message when explicit messages are used
• rejects prompt-construction options that conflict with explicit message histories
• adds focused CLI tests for JSON string, file, stdin, -M, prompt appending, and conflict handling

Impact

Users can replay or extend structured message histories from the CLI using the same serialized message format supported by the Python API.

Validation

• uv run pytest tests/test_cli_messages.py
• uv run pytest tests/test_cli_messages.py tests/test_parts.py tests/test_llm.py
• uv run pytest