rewind_conversation internals refactored into shared helper functions

The handler for the rewind_conversation remote request - which rewinds a conversation to an earlier point - has been restructured. The logic for persisting the rewind, checking whether the session state has drifted, mirroring and logging failures, and stopping any in-flight agents past the cut point now lives in separate helper functions instead of being written inline. Stopping in-flight agents is now driven by a cut index rather than the previous predicate-based walk through agent ancestry.