filesystem: add descriptions to all tool parameters

[m9ter1a]

Several tools in the filesystem server expose parameters — path, content, source, destination, pattern, excludePatterns — with no description. When an agent fills these in it has only the parameter name to go on; path, for instance, doesn't convey that it must resolve inside one of the server's allowed directories.

This PR adds a short .describe(...) to each previously-undescribed parameter (18 in total). It's a metadata-only change:

• no behavior change; all 152 existing tests pass (vitest run)
• descriptions are brief and factual — path fields note the allowed-directory constraint, and so on

I noticed the gap while auditing MCP servers' tool schemas for agent-friendliness with mcpcheck.