cgc watch does not respect .cgcignore rules #716

Description

cgc watch does not apply .cgcignore filtering, causing unnecessary graph updates when files in ignored directories change.

While build_graph_from_path_async() in graph_builder.py correctly searches for .cgcignore, parses it with pathspec, and filters files before indexing, the RepositoryEventHandler in core/watcher.py bypasses this entirely.

Steps to Reproduce

1. Create a .cgcignore file in a project root with patterns like:

   **/bin/
   **/obj/
   

2. Run cgc index — files in bin/ and obj/ are correctly excluded.
3. Run cgc watch in the same directory.
4. Trigger a build (e.g., dotnet build for a .NET project, or any build tool that writes to bin//obj/).
5. Observe that the watcher receives events for files in ignored directories and triggers full repository refreshes.

Root Cause

In core/watcher.py, the RepositoryEventHandler event handlers (on_created, on_modified, on_deleted, on_moved) only check file extensions via self.graph_builder.parsers:

def on_modified(self, event):
    if not event.is_directory and Path(event.src_path).suffix in self.graph_builder.parsers:
        self._debounce(event.src_path, lambda: self._handle_modification(event.src_path))

There is no .cgcignore check here. Additionally, _handle_modification() and _initial_scan() both use a bare rglob("*") without .cgcignore filtering:

all_files = [f for f in self.repo_path.rglob("*") if f.is_file() and f.suffix in supported_extensions]

This means:

1. Events for ignored files trigger processing — e.g., a .cs file generated in obj/ triggers _handle_modification().
2. The re-scan includes ignored files — the full repository refresh parses files that cgc index would have skipped.

Expected Behavior

• The watcher should load .cgcignore (same logic as build_graph_from_path_async) and:
  1. Skip events for files matching ignore patterns in on_created/on_modified/on_deleted/on_moved.
  2. Apply the same pathspec filtering in _initial_scan() and _handle_modification() when collecting files via rglob.

Impact

For projects with build artifacts in ignored directories (common in .NET, Java, Node.js, etc.), every build triggers a flood of watcher events and unnecessary full graph re-scans. In my testing with a .NET project, a single dotnet build generated tens of thousands of fsevents from bin/ and obj/ directories.

Suggested Fix

Pass the loaded pathspec.PathSpec into RepositoryEventHandler.__init__() and:

1. Add a _should_ignore(self, path: str) -> bool method.
2. Check it in each event handler before debouncing.
3. Apply the same filter in _initial_scan() and _handle_modification() when building the file list.