rewind_conversation's target-validation logic pulled into a single shared helper
The code behind rewind_conversation (which lets a remote or headless connection jump the conversation back to an earlier message) has had its target-validation logic consolidated into one helper function. That helper now handles finding the target message, checking for stale or unseen targets, handling poll-tool-result targets, and checking delivered poll events within range, returning either a refusal or a description of what to cut. Other nearby code for re-homing the working directory over MCP and for handling messages/poll events was renamed but not otherwise changed.
This further consolidates rewind's target-finding logic into a single, more maintainable helper, continuing earlier work to share this logic rather than adding new capability for users.