pi-mono Deep Dive 5: Session Tree — Append-only JSONL, Branching Without History Mutation, Compaction Logic Full Analysis
SessionManager core: JSONL append-only storage, id/parentId tree formation, branch() moves leaf pointer without mutating history, buildSessionContext() handles compaction entries, createBranchedSession() forks to new file. Complete Entry types: message, thinking_level_change, model_change, compaction, branch_summary, custom, custom_message, label, session_info. Migration v1→v2→v3 details.