Add --watch flag that polls a file and redraws the terminal when its
content changes. Useful for monitoring log files, configuration files,
or any file that changes over time.
Features:
- Polls file every 1 second for changes
- Disables pager (pager would block the watch loop)
- File-only: rejects stdin or - input
- Error recovery: retries when file is missing or temporarily unreadable
- State comparison: only redraws when content actually changes
Thread RP_DEBUG-driven logging through input, detection, rendering, and pager decisions so CLI behavior is inspectable without polluting stdout. Add focused coverage for debug wiring, env parsing, and stderr-only output.
