Add .coderabbit.yaml configuration for automated code review via CodeRabbit.

Includes:

• Review path instructions for all major code areas (modules, module_utils, plugins, tests, CI)
• Docstring generation guidelines with path-specific instructions (rST for config/facts, YAML blocks for modules, skip for auto-generated argspec)
• Unit test generation guidelines with separate patterns for resource modules (full state matrix, dual get_resource_connection mocking) and legacy modules (run_commands mocking)
• Tool configuration (gitleaks, semgrep, yamllint, actionlint enabled; eslint/biome/ruff disabled)
• Schema-validated against CodeRabbit v2 schema