research(core): session persistence with replay, resume, and fork capabilities

[bug-ops]

OpenAI Codex CLI persists every session as rollout files (JSONL.zst) enabling resume, fork, and compaction+resume workflows. Zeph has SQLite message history and /compact but no session-level resume/fork. Proposed: session serialization format, CLI/TUI commands (session list/resume/fork), integration with existing compaction. References: https://deepwiki.com/openai/codex/3.3-session-management-and-persistence