#806 Stage 7 epic: fork / time-travel (#agentic)

POST /workflows/<id>/fork (+ MCP fork_run): new run of same definition; copy variable+checkpoint rows up to turn boundary N (keep agent_llm_<k>/agent_tool_<k>_<i>/agent_answer_<k> for k<N; truncate agent_messages; reset agent_done/agent_turn_idx/agent_post_fanout); apply optional edits; enqueue. Do NOT copy token_usage/run_budget (fresh budget), parallel_join_state, pending approvals (keys embed run_id -> fork re-requests). Optional forked_from_run_id column for lineage. HIGHEST determinism risk. Gates: golden demos + harness fork_diverges.