AtomicWriteFile fails with EBUSY when target is a bind-mounted file (breaks claude-code client in Flatpak)

Bug description

When ToolHive is run inside a Flatpak sandbox with --filesystem=~/.claude.json granted, registering the claude-code client fails with EBUSY. This is caused by AtomicWriteFile (pkg/fileutils/atomic.go) using a write-to-temp-then-rename pattern. When Flatpak grants access to an individual file path, bubblewrap bind-mounts that specific file as a mount point inside the sandbox namespace. rename() onto a mount point returns EBUSY on Linux, so the atomic replacement fails.

claude-code is the only affected client because it is the only one whose config file lives directly in the home directory root (~/.claude.json, RelPath: []string{}). All other clients use subdirectory paths, so Flatpak grants directory-level access - both the temp file and the rename target live inside the same bind-mounted directory, where rename works freely.

Steps to reproduce

1. Package an app that invokes thv inside a Flatpak sandbox (ToolHive Desktop does this, or you can vibe code a minimal sample)
2. Grant --filesystem=~/.claude.json in the Flatpak manifest
3. Ensure ~/.claude.json exists on the host (e.g. Claude Code is installed)
4. Run thv client register claude-code with at least one MCP server running. Important: it has to be running inside the Flatpak environment

Probably it can be reproduced also with bubblewrap directly, as there should be nothing Flatpak-specific involved. So instead of bundling a whole flatpak package, you can probably just run thv using bubblewrap directly, and create the same mount.

Expected behavior

The claude-code client is registered and ~/.claude.json is updated with the running MCP server entries.

Actual behavior

The write fails with:

rename /home/$USER/.tmp-2168705275 /home/$USER/.claude.json: device or resource busy

Environment (if relevant)

• OS/version: Linux (any distro running Flatpak with bubblewrap)
• ToolHive version: any

Additional context

The same issue would affect any other sandboxed environment that bind-mounts individual files, such as Docker with a specific file volume mount (-v /host/file:/container/file).

Start with pkg/fileutils/atomic.go and inspect AtomicWriteFile's temporary-file and rename flow. Reproduce the failure with a specific-file bind mount using bubblewrap, then verify the behavior against the expected claude-code registration result. Done means registration updates ~/.claude.json successfully without EBUSY while preserving the existing atomic-write behavior where possible.