Complexity scoring — go beyond line count

## Feature Request

Add complexity-aware scoring that goes beyond raw line counts to better estimate review effort.

### Proposed signals

- **New files vs modifications** — new files require more review context than edits to existing ones
- **Number of distinct directories touched** — cross-cutting changes are harder to review
- **File type mix** — PRs spanning frontend + backend + infra are more complex than single-layer changes
- **Churn density** — many small changes across many files vs focused changes in few files

### Proposed behavior

- Compute a complexity score alongside the line count
- Use the complexity score (or a blend of both) to determine reviewer count
- Expose the score as an output (`complexity-score`)
- Show the breakdown in the PR comment

### Why

A 200-line PR touching one file is very different from a 200-line PR touching 15 files across 8 directories. Complexity scoring captures this nuance.