Review code changes for bugs, security issues, and code quality problems.

## Modes
- `staged` - Review staged git changes (default)
- `branch:NAME` - Compare current HEAD with a branch (e.g., `branch:main`)
- `commit:SHA` - Review a specific commit
- `files:path1,path2` - Review specific files

## What it checks
- **Bugs**: Logic errors, null checks, race conditions, off-by-one errors
- **Security**: Injection attacks, hardcoded secrets, insecure patterns
- **Performance**: N+1 queries, memory leaks, inefficient algorithms
- **Quality**: Missing error handling, poor naming, duplication

## Output
Returns actionable feedback with severity, location, and fix suggestions.

## Examples

Review staged changes:
```json
{"mode": "staged"}
```

Compare with main branch:
```json
{"mode": "branch:main"}
```

Review specific files:
```json
{"mode": "files:src/auth.py,src/api.py"}
```
