OIC provider adapter for OpenAI Codex #236

Context

A second provider tests whether the abstraction really works. Start only after the MVP (#235) passes with Claude Code.

Behavior to implement

• An adapter from Codex's execution interface to AgentProvider (OIC agent provider interface, with a fake provider for tests #222), including resume-from-checkpoint when sessions can't continue.
• Run the same lifecycle test suite against both providers.

Acceptance criteria

• The MVP scenario passes with Codex with no changes to domain or service code.