feat: add shallow/deep review depth presets (default: shallow with kimi-k2-0711)

[factory-nizar]

Summary

Adds a review_depth input to control review thoroughness. This makes it easy for users to switch between fast and thorough reviews.

Review Depth Presets

Depth	Model	Reasoning Effort	Best For
deep (default)	gpt-5.2	high	Thorough reviews catching subtle bugs and design issues
shallow	kimi-k2-0711	default	Fast, cost-effective reviews for straightforward PRs

How to use

# Deep review (default - no extra config needed)
- uses: Factory-AI/droid-action@v3
  with:
    factory_api_key: ${{ secrets.FACTORY_API_KEY }}
    automatic_review: true

# Shallow review for faster feedback
- uses: Factory-AI/droid-action@v3
  with:
    factory_api_key: ${{ secrets.FACTORY_API_KEY }}
    automatic_review: true
    review_depth: shallow

# Fully custom model (overrides depth preset entirely)
- uses: Factory-AI/droid-action@v3
  with:
    factory_api_key: ${{ secrets.FACTORY_API_KEY }}
    automatic_review: true
    review_model: claude-sonnet-4-5-20250929
    reasoning_effort: high

Changes

• New review_depth input (shallow/deep, default: deep) on both action.yml and review/action.yml
• New src/utils/review-depth.ts utility with centralized depth presets (easy to modify)
• review_model and reasoning_effort defaults changed from hardcoded values to empty (derived from depth)
• Explicit review_model/reasoning_effort overrides still take priority over depth presets
• README updated with review depth documentation, usage examples, and instructions for updating model selections

Resolves: FAC-17934

[factory-droid]

Droid finished @factory-nizar's task —— View job

---

[jonathan-factory]

Can maybe use an enum:

export enum ReviewDepth {
  Shallow = 'shallow',
  Deep = 'deep',
}