Improve directory watching for macOS compatibility

[mixberrymc]

Description

Fixes the regolith watch command failing on macOS with the error:

[ERROR] Could not start watching `./packs/RP`
[ERROR] lstat packs/RP/...: no such file or directory

Problem

The watch command was using the experimental "..." recursive watching syntax (filepath.Join(root, "...")) from the fsnotify fork, which doesn't work reliably on macOS.

Solution

• Replaced the "..." syntax with filepath.WalkDir to manually traverse and add each subdirectory to the watcher
• Added the os import for os.DirEntry
• This approach provides better cross-platform compatibility

Testing

Tested on:

• macOS (darwin/amd64) - ✅ Watch command now works
• The fix maintains compatibility with the existing watcher behavior on other platforms

Changes

• Modified regolith/watcher.go:
  • Updated watch() function to use filepath.WalkDir
  • Added os to imports