fix(filesystem): note allowed-directories scope on read_file

[yash161004]

What

Adds the access-boundary sentence to read_file's description in the filesystem
server, matching every other tool it ships with.

-    description: "Read the complete contents of a file as text. DEPRECATED: Use read_text_file instead.",
+    description: "Read the complete contents of a file as text. DEPRECATED: Use read_text_file instead. Only works within allowed directories.",

Why

All 14 filesystem tools end their description with "Only works within allowed
directories." — except read_file, the deprecated alias:

Tool	States the boundary
read_file	no
read_text_file, read_media_file, read_multiple_files, write_file, edit_file, create_directory, list_directory, list_directory_with_sizes, directory_tree, move_file, search_files, get_file_info, list_allowed_directories	yes

read_file is registered with ReadTextFileArgsSchema.shape and dispatches to
readTextFileHandler — the same schema and the same handler as
read_text_file. The boundary is therefore enforced identically; the two differ
only in how they are described.

This is a documentation gap, not a security issue. Path validation is
unchanged and works correctly. Nothing here suggests unrestricted access.

It's worth fixing because the description is the part an LLM agent reasons over
when selecting a tool. read_text_file tells the model it is constrained to
allowed directories; its alias, backed by the identical code path, does not — so
the same operation is presented to the model two different ways. Restating the
constraint costs one sentence and removes the discrepancy.

Notes

• One-line change to a string literal; no behaviour, schema, or handler changes.
• No test asserts this string (it appears exactly once in the tree).
• Consistent with the recent metadata tidy-up in fix(filesystem): declare openWorldHint on all tools #4480, which brought
  openWorldHint into line across the same tool set.

_{Found while auditing MCP tool definitions for description/behaviour drift.
Prepared with assistance from Claude.}