Adds session context management to prevent session poisoning: - CONTEXT_DIR and ENABLE_CONTEXT_DUMP config options - issue_ref_to_context_file() - derive context file path - kugetsu_context_load() - load previous context - kugetsu_context_dump() - save context on session start - kugetsu_context_update_message() - append to conversation history - Integration in cmd_start and cmd_continue - New 'kugetsu context' command