New permissions.blockReadsOutsideWorkingDirectories setting, with network-path detection for working directories

A new setting, permissions.blockReadsOutsideWorkingDirectories, is now read from settings and propagated through settings serialization/merge. When enabled it switches directory validation to a stricter check (kHt(n) instead of fS(n)), adds blockReadsOutsideWorkingDirectories: true to the tool permission context, and is enforced in the sandbox filesystem policy, command-parsing/read-permission logic, and user-facing deny/ask messages ("Reads outside the working directories are blocked"). It is explicitly stripped out for remote-execution permission modes.

Alongside it, validating a directory to add as a trusted working directory (validateDirectoryForWorkspace) now checks whether the path is a network path before doing a normal stat, returning a new networkPath result type instead of treating it as an ordinary directory-check failure. This rejects UNC/network paths from being added as working directories, with guidance about drive-letter mapping on Windows.