fix: validate IDs in GetNextCheckpointSequence before directory read

The PostToolUse (TodoWrite) hook passed input.SessionID and the task tool-use ID straight into SessionMetadataDirFromSessionID / TaskMetadataDir and then os.ReadDir, with no validation — unlike every other hook entry point. A crafted "../.." could redirect the directory listing. Validate both IDs at this choke point; an invalid value simply starts the checkpoint sequence at 1.