fix: gracefully handle permission errors in run_watch

[echobt]

Description

This PR addresses issue #194 by adding checks for path existence and read permissions before initializing the file watcher in run_watch.

Changes

• Added a check if the path exists.
• Added a check if the path is readable (by attempting read_dir).
• Used anyhow::bail! to return clear error messages to the user instead of propagating generic IO errors or crashing later.

Testing

• Verified with non-existent directory -> Correctly reports "Path does not exist".
• Verified with existing directory -> Proceed to watch.
• (Implicitly) Permission denied on read_dir will now report "Failed to access directory ... Check permissions.".

Checklist

• Code compiles
• Tests pass
• Manual verification