feat(mcp): add Windows support for code search via PowerShell

[noctrex]

Replace grep with PowerShell Select-String on Windows for the search_code and search_graph tools, enabling cross-platform functionality.

• Use tab-delimited output from Select-String to avoid conflicts with colons in Windows drive-letter paths (e.g. C:\dir\file).
• Update collect_grep_matches to parse delimiter as tab on Windows and colon on POSIX.
• Open scoped filelist in binary mode ("wb") and use forward slashes on Windows to prevent xargs from interpreting backslashes as escape sequences.
• Use cbm_tmpdir() for pattern temp files instead of hardcoded /tmp/.
• Define getpid as _getpid on Windows for consistent API.

Fixes #196