chore: add CodeRabbit configuration for automated code review

[christian-byrne]

Adds a minimal .coderabbit.yaml configuration to enable AI-powered code review via CodeRabbit.

What this enables

• Inline code review comments on PRs with actionable feedback
• Ruff integration — uses existing pyproject.toml config, no duplication
• Gitleaks — secret detection to catch accidental API key commits
• GitHub checks integration — references CI results for context
• Path-specific review guidance — tailored focus areas for comfy/, comfy_api_nodes/, comfy_extras/, comfy_execution/, nodes.py, and alembic_db/
• Knowledge base learnings — improves over time from resolved comments
• AI agent prompts — inline fix instructions for AI coding tools

What this does NOT do (kept minimal to avoid spam)

• ❌ No PR description/title modifications
• ❌ No summary comments or walkthrough posts
• ❌ No label suggestions or auto-labeling
• ❌ No reviewer suggestions or auto-assignment
• ❌ No poems, fortunes, sequence diagrams, or effort estimates
• ❌ No auto-generated docstrings or unit tests
• ❌ No request-changes workflow

Excluded from review

• comfy_api_nodes/apis/** (auto-generated Pydantic models)
• **/generated/*.pyi (auto-generated type stubs)
• .ci/**, script_examples/**, images, notebooks, batch scripts

Profile

Set to chill to minimize noise — important since this repo requires conversation resolution before merge (unresolved CodeRabbit threads would block merging).

[Kosinkadink]

Let's give this a whirl since we got approval