feat(native): filter Ghostty logs (#1426)

## Summary
- recognize the selected Ghostty `std.log` scopes in OpenTUI's root
logger
- keep all standard-library logs silent by default
- read `OTUI_GHOSTTY_LOG_LEVEL` inside the native `logFn` and forward
enabled messages through the existing `logger.logMessage` path
- support `error`/`err`, `warn`/`warning`, `info`, and `debug`
thresholds
- document the environment variable

## Behavior
Unset or invalid `OTUI_GHOSTTY_LOG_LEVEL` values emit no `std.log`
messages. Recognized Ghostty scopes are forwarded up to the configured
severity. Logs from every other scope are discarded. Explicit OpenTUI
logger calls are unchanged.

## Testing
- release `bun run build` from `packages/core`
- `bun run test:native` from `packages/core` (2066 passed, 8 skipped)
- `zig fmt --check packages/native/src/lib.zig
packages/native/src/logger.zig`
- `oxfmt --check packages/web/src/content/docs/reference/env-vars.mdx`