filesystem: document hard-link behavior and report the link count in get_file_info #5226

The filesystem server's README should spell out what hard links mean for reads and writes, and get_file_info should report a file's link count.

Problem

write_file and edit_file overwrite an existing file in place, and the README says so (src/filesystem/README.md:106: the file "keeps its inode, creation time, hard links and permissions"). It does not spell out the consequence: when a file inside an allowed directory is a hard link to a file elsewhere, both names are the same file, so an overwrite changes the content seen at every name, and a read returns that shared content. Some tools create hard links routinely (pnpm's content-addressable store, for example, hard-links package files into node_modules), so this can happen without anyone choosing it.

The server cannot currently tell a caller whether a file has other names: get_file_info does not report the link count.

Expected

1. A README paragraph, under write_file (or in a short note on allowed directories), saying that the server's allowed-directory checks are path-based, that a hard link is a real name of the same file, and that overwriting or editing such a file changes it at all of its names.
2. get_file_info reports the link count (nlink from fs.stat), so a client or model can see that a file has more than one name before changing it.

Tests for the new get_file_info field, and a changeset (minor: a new output field).