fix(filesystem): reject existing move destinations

[dev-willbird1936]

Description

Make the filesystem server's move_file honor its documented no-overwrite contract. Existing destination files, directories, and symlinks now return a tool error instead of being replaced.

Root cause

POSIX fs.rename replaces an existing destination. The previous implementation could therefore destroy destination data despite the tool description promising failure on conflicts.

Testing

• Fresh filesystem recheck on 2026-07-15: focused regression 11/11 and full filesystem suite 153/153 passed.
• Filesystem package build passed.
• git diff --check passed.
• Earlier validation recorded the full workspace test/build result; unrelated workspace packages were not rerun in this fresh recheck.
• LLM-client integration was not exercised; tests call the server tool implementation directly.

The repository baseline itself fails a default Prettier check for these two existing files; the same check fails on origin/main. No unrelated formatter churn is included in this branch. Hosted CI and maintainer review remain pending.