feat: Add support for custom global file exclusion patterns

[akramcodez]

Description

Users cannot define their own global file exclusion patterns. The CLI only supports hardcoded defaults (like node_modules, .git) and per-project .blackboxignore files.

Current Behavior

• No way to add custom global patterns
• getCustomExcludes() method exists but returns empty array

Expected Behavior

Users should be able to define global exclusion patterns in their config (e.g., in settings.json) that apply across all projects.

Use Case

Users who want to always exclude:

• Build artifacts (dist/, build/)
• OS files (.DS_Store)
• Personal cache folders
• Language-specific compiled files (*.pyc, __pycache__)