Refactor AI session handling for Codex provider

- Add `error_summary` field to `AiSession` for sessions without user messages
- Implement functions to read and process Codex session files:
  - `read_codex_context_since`, `read_codex_last_context`, `read_codex_exchanges`
- Enhance session parsing with `parse_codex_session_file` to extract metadata
- Update session retrieval functions to handle `Provider::Codex`
- Introduce directory helpers for Codex sessions (`get_codex_sessions_dir`)
- Improve extraction of messages and error summaries from Codex entries