dsh-session-repair: Fix DSH session logs caused by unknown event types

dsh-session-repair is a plugin for DeepSeek Harness designed to address issues in `dsh 0.1.0-rc.x` where external plugins writing unknown events cause `SessionFormatUnsupportedError` upon service restart and prevent session logs from opening, due to the lack of a third-party event type registration mechanism. The tool primarily offers two core functions: scanning zstd-compressed session logs via `action=scan` to report unknown event types and their repairability (defaulting to read-only mode without modifications); and repairing non-live session logs using `action=repair` or `repair_id` to add missing `ignorable` flags for unknown events. To ensure reliability...