fix: improve error messages in gen command

[pageton]

Summary

Error messages in the CLI have awkward or redundant phrasing:

1. internal/gen/gen.go:32

return fmt.Errorf("error render template got error: %v", err)

Grammatically awkward and redundant ("error...got error"). Should be:

return fmt.Errorf("code generation failed: %w", err)

2. internal/gen/gen.go:27

return fmt.Errorf("error processing %s: %v", input, err)

Acceptable but uses %v instead of %w for error wrapping. Should be:

return fmt.Errorf("processing %s: %w", input, err)

3. internal/gen/generator.go:192

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