dsh-single-instance-guard: DSH Single-Instance Lock Plugin

This plugin aims to prevent session log corruption in DeepSeek Harness (DSH) caused by concurrent startup. By creating an atomic exclusive lock file (`.dsh-server.lock`) in the `DSH_HOME` directory, it ensures that only one server instance runs in that directory. If a conflict is detected at startup, the plugin either aborts with an error or cleans up an expired lock based on the liveness of the running process. The lock file is automatically removed upon process exit. With zero dependencies and support for CLI installation and manual configuration, the plugin effectively prevents log corruption errors such as "seq gap".