Adds two file manipulation tools to the filesystem server:

modify_file

• Find and replace text in files using string matching or regex
• Supports single or multiple occurrences replacement
• Returns modification count and file metadata

search_within_files

• Search for text within file contents across directory trees
• Configurable depth limits and result caps
• Excludes binary files, reports line numbers and context
• Distinct from existing search_files which only searches filenames