Internal refactor of rewind_conversation target validation in the remote/headless bridge

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.