Add a "watch" mode for continuous validation

[kehoecj]

Description

Add a --watch flag that monitors the search path for file changes and re-validates automatically. This is useful during local development when editing config files.

Proposed behavior

# Watch current directory, re-validate on changes
validator --watch .

# Combine with other flags
validator --watch --gitignore --file-types=json,yaml .

• On startup, run a full validation pass
• Watch for file create/modify/rename events in the search path
• On change, re-validate only the changed file(s)
• Respect all existing filters (--exclude-dirs, --gitignore, --file-types, etc.)
• Print a separator between runs for readability
• Exit on Ctrl+C

Implementation notes

• Use fsnotify for cross-platform file watching (well-maintained, pure Go, used by Hugo/Viper/etc.)