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/workflows/codeql.yml

@@ -0,0 +1,41 @@
+name: CodeQL Security Scan
+
+on:
+  push:
+    branches: [ main, master, develop ]
+  pull_request:
+    branches: [ main, master ]
+  schedule:
+    - cron: '0 0 * * 1'  # Weekly on Monday
+
+jobs:
+  analyze:
+    name: Analyze
+    runs-on: ubuntu-latest
+    permissions:
+      actions: read
+      contents: read
+      security-events: write
+
+    strategy:
+      fail-fast: false
+      matrix:
+        language: [ 'python' ]
+
+    steps:
+    - name: Checkout repository
+      uses: actions/checkout@v4
+
+    - name: Initialize CodeQL
+      uses: github/codeql-action/init@v2
+      with:
+        languages: ${{ matrix.language }}
+        queries: security-extended,security-and-quality
+
+    - name: Autobuild
+      uses: github/codeql-action/autobuild@v2
+
+    - name: Perform CodeQL Analysis
+      uses: github/codeql-action/analyze@v2
+      with:
+        category: "/language:${{matrix.language}}"