Add comprehensive GitHub Actions CI/CD workflows

Implemented automated testing and quality assurance pipelines:

Workflows added:
- python-package.yml: Main CI pipeline
  * Multi-OS testing (Ubuntu, Windows, macOS)
  * Python 3.9-3.12 compatibility testing
  * Linting with flake8
  * Code formatting checks with black
  * Type checking with mypy
  * Import and compilation tests
  * Unit tests for core components

- codeql.yml: Security scanning
  * Weekly automated security analysis
  * CodeQL vulnerability detection
  * Extended security and quality queries

- dependency-review.yml: Dependency safety
  * Automatic dependency vulnerability checks
  * License compliance verification
  * Blocks moderate+ severity issues

- pr-labeler.yml: PR automation
  * Auto-labels based on changed files
  * PR size labeling (xs/s/m/l/xl)
  * Metadata extraction for better organization

- welcome.yml: Community engagement
  * Welcomes first-time contributors
  * Provides helpful guidelines
  * Improves contributor experience

Configuration:
- labeler.yml: Label mapping for automatic categorization

All workflows include proper permissions and error handling.

.github/labeler.yml

@@ -0,0 +1,41 @@
+documentation:
+  - '**/*.md'
+  - '**/*.txt'
+  - 'docs/**/*'
+
+dependencies:
+  - 'requirements.txt'
+  - 'setup.py'
+  - 'pyproject.toml'
+
+configuration:
+  - '**/*.yml'
+  - '**/*.yaml'
+  - '**/*.json'
+  - '**/*.toml'
+  - '.env.example'
+
+core:
+  - 'chatbot.py'
+
+gui:
+  - 'gui.py'
+
+api:
+  - 'ai_provider.py'
+
+memory:
+  - 'memory.py'
+
+config:
+  - 'config.py'
+
+utilities:
+  - 'utils.py'
+
+ci/cd:
+  - '.github/workflows/**/*'
+
+tests:
+  - 'test_*.py'
+  - 'tests/**/*'