docs(filesystem): clarify argless form behavior (Fixes #4423)

[TomrroW007]

Description

This pull request updates the src/filesystem/README.md documentation to clarify server startup behavior and configuration auditing for directory access control.

Documentation improvements:

• Added a note explaining that starting the server without path arguments delegates directory access control to the client at runtime, which can hinder auditability. It recommends specifying allowed directories as command-line arguments for a self-contained and auditable configuration.

(Note: Left the registry publishing note intact as per standard template instructions)

Server Details

• Server: filesystem
• Changes to: documentation (README.md)

Motivation and Context

Fixes #4423.

When the filesystem server is launched without path arguments, it correctly relies on the client's roots protocol for scope verification at runtime. However, this behavior makes the access boundary invisible at the config level. This PR adds a note to clarify this behavior and guides users who want an auditable, self-contained config to use explicit path arguments.

How Has This Been Tested?

N/A - This is purely a documentation update.

Breaking Changes

None. Users will not need to update their MCP client configurations.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Protocol Documentation
• My changes follows MCP security best practices
• I have updated the server's README accordingly
• I have tested this with an LLM client
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have documented all environment variables and configuration options

Additional context

N/A

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