fix: improve error messages in gen command

## Summary

Error messages in the CLI have awkward or redundant phrasing:

### 1. `internal/gen/gen.go:32`
```go
return fmt.Errorf("error render template got error: %v", err)
```
Grammatically awkward and redundant ("error...got error"). Should be:
```go
return fmt.Errorf("code generation failed: %w", err)
```

### 2. `internal/gen/gen.go:27`
```go
return fmt.Errorf("error processing %s: %v", input, err)
```
Acceptable but uses `%v` instead of `%w` for error wrapping. Should be:
```go
return fmt.Errorf("processing %s: %w", input, err)
```

### 3. `internal/gen/generator.go:192`
```go
return fmt.Errorf("invalid interface %s: %w", iface.Name, iface.err)
```
This one is fine — shown for comparison of the preferred style.

## Why

- Consistent error message style across the codebase
- Using `%w` enables callers to use `errors.Is`/`errors.As` for programmatic error handling
- Removing redundant "error...error" phrasing makes logs easier to scan