### What's wrong?

When using Neovim inside a Herdr workspace, files modified externally by a coding agent can remain stale in the open Neovim buffer after switching back from the agent pane.

The file on disk has already changed, but Neovim continues to display the previous buffer contents until `:checktime` or `:edit!` is run manually.

  ## To reproduce

  1. Start a Herdr workspace.
  2. Open Neovim in the workspace with `nvim .`.
  3. Open a file in Neovim.
  4. Start Codex or another coding agent in a different Herdr pane.
  5. Ask the agent to modify the open file.
  6. Switch back to the Neovim pane.

  ## Actual behavior

The Neovim buffer still shows the old contents, even though the file on disk has changed.

  `:set autoread?` reports that `autoread` is enabled.

Running this command manually immediately updates the buffer:

 ```vim
  :checktime
```

The issue occurs when switching Herdr panes because Neovim does not appear to receive a FocusGained event.

  ## Expected behavior

When the user returns to the Neovim pane, externally modified files should be detected and reloaded automatically when the buffer has no unsaved local changes.

Buffers with local unsaved changes should remain protected from automatic overwrites.

### System details
- Omarchy Quattro 4.0.0-1
- omarchy-nvim 2026.8.13-1
- herdr 0.8.0.r13-1
- Coding agent: Codex (version 0.149.1) 

## Additional context

Omarchy Quattro ships Neovim with LazyVim and includes Herdr as the agent workspace manager.

The default LazyVim checktime autocmds use FocusGained, TermClose, and TermLeave, but switching between Herdr panes does not reliably trigger FocusGained for Neovim.

A possible fix would be to also check for external changes on BufEnter and/or CursorHold, while skipping buffers with modified set.