feat: fork pattern v2

[jif-oai]

Adds this:

properties.insert(
            "fork_turns".to_string(),
            JsonSchema::String {
                description: Some(
                    "Optional MultiAgentV2 fork mode. Use `none`, `all`, or a positive integer string such as `3` to fork only the most recent turns."
                        .to_string(),
                ),
            },
        );
        ```

[jif-oai]

@codex review

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Apply rollback to user turns, not all fork boundaries

ThreadRolledBackEvent.num_turns is defined as a count of user turns, but fork_turn_positions_in_rollout truncates a list that also contains trigger_turn assistant envelopes. In histories with trigger envelopes, a rollback can remove the wrong boundary, so truncate_rollout_to_last_n_fork_turns picks an incorrect keep_idx and forks stale/incorrect context.

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