powershell-fix: Check and fix Windows PowerShell commands in DSH

powershell-fix is a host layer plugin for DeepSeek Harness (DSH), aiming to solve failure issues caused by bash habits, paste pollution (e.g., CRLF, prompts), and continuation character errors when agents execute commands in the Windows PowerShell environment. The plugin provides the `powershell_fix` tool, which first checks and fixes commands locally before executing them via the host shell pathway. It follows DSH's existing sandbox, approval, and timeout policies throughout the process without adding new permissions. Core features include: normalizing characters and removing prompts; converting bash continuation characters to PowerShell backticks; issuing warnings instead of blindly modifying commands that cannot be safely rewritten; and built-in execution gates to block dangerous commands such as recursive deletion and disk formatting.