ci: add worker validation and Docker build checks

Add automated validation to prevent worker-related issues:

**Worker Validation Script:**
- New script: .github/scripts/validate-workers.sh
- Validates all workers in docker-compose.yml exist
- Checks required files: Dockerfile, requirements.txt, worker.py
- Verifies files are tracked by git (not gitignored)
- Detects gitignore issues that could hide workers

**CI Workflow Updates:**
- Added validate-workers job (runs on every PR)
- Added build-workers job (runs if workers/ modified)
- Uses Docker Buildx for caching
- Validates Docker images build successfully
- Updated test-summary to check validation results

**PR Template:**
- New pull request template with comprehensive checklist
- Specific section for worker-related changes
- Reminds contributors to validate worker files
- Includes documentation and changelog reminders

These checks would have caught the secrets worker gitignore issue.

Implements Phase 1 improvements from CI/CD quality assessment.

.github/pull_request_template.md

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+## Description
+
+<!-- Provide a brief description of the changes in this PR -->
+
+## Type of Change
+
+<!-- Mark the appropriate option with an 'x' -->
+
+- [ ] 🐛 Bug fix (non-breaking change which fixes an issue)
+- [ ] ✨ New feature (non-breaking change which adds functionality)
+- [ ] 💥 Breaking change (fix or feature that would cause existing functionality to not work as expected)
+- [ ] 📝 Documentation update
+- [ ] 🔧 Configuration change
+- [ ] ♻️ Refactoring (no functional changes)
+- [ ] 🎨 Style/formatting changes
+- [ ] ✅ Test additions or updates
+
+## Related Issues
+
+<!-- Link to related issues using #issue_number -->
+<!-- Example: Closes #123, Relates to #456 -->
+
+## Changes Made
+
+<!-- List the specific changes made in this PR -->
+
+-
+-
+-
+
+## Testing
+
+<!-- Describe the tests you ran to verify your changes -->
+
+### Tested Locally
+
+- [ ] All tests pass (`pytest`, `uv build`, etc.)
+- [ ] Linting passes (`ruff check`)
+- [ ] Code builds successfully
+
+### Worker Changes (if applicable)
+
+- [ ] Docker images build successfully (`docker compose build`)
+- [ ] Worker containers start correctly
+- [ ] Tested with actual workflow execution
+
+### Documentation
+
+- [ ] Documentation updated (if needed)
+- [ ] README updated (if needed)
+- [ ] CHANGELOG.md updated (if user-facing changes)
+
+## Pre-Merge Checklist
+
+<!-- Ensure all items are completed before requesting review -->
+
+- [ ] My code follows the project's coding standards
+- [ ] I have performed a self-review of my code
+- [ ] I have commented my code, particularly in hard-to-understand areas
+- [ ] I have made corresponding changes to the documentation
+- [ ] My changes generate no new warnings
+- [ ] I have added tests that prove my fix is effective or that my feature works
+- [ ] New and existing unit tests pass locally with my changes
+- [ ] Any dependent changes have been merged and published
+
+### Worker-Specific Checks (if workers/ modified)
+
+- [ ] All worker files properly tracked by git (not gitignored)
+- [ ] Worker validation script passes (`.github/scripts/validate-workers.sh`)
+- [ ] Docker images build without errors
+- [ ] Worker configuration updated in `docker-compose.yml` (if needed)
+
+## Screenshots (if applicable)
+
+<!-- Add screenshots to help explain your changes -->
+
+## Additional Notes
+
+<!-- Any additional information that reviewers should know -->