fix: ignore nested dependency trees in watcher - #4

Fixes #5

Summary

Prevent /diff-review from crashing Pi when reviewing repositories that contain nested node_modules or .git
directories.

Problem

The file watcher only ignored .git and node_modules when they were located directly under the repository root.

In repositories containing nested dependency trees or embedded Git repositories, Chokidar could create too many file
watchers and eventually emit:

 EMFILE: too many open files, watch

Because the watcher error was unhandled, the error terminated Pi.

Changes

• Ignore .git and node_modules directories at any depth.
• Handle watcher errors so they do not become uncaught exceptions.
• Add regression tests for nested ignored directories.