Add watch mode for live file monitoring

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

README.md

@@ -9,6 +9,7 @@ Rich print files — markdown, JSON, code, and more — beautifully in your term
 - **JSON formatting** with syntax highlighting
 - **Pager integration** with mouse scrolling support
 - **Stdin support** for piping content
+- **Watch mode** for live file redraws
 
 ## Installation
 
@@ -65,6 +66,15 @@ rp --no-pager file.py     # Disable pager
 
 The pager prefers `less -R --mouse` for mouse scrolling when available, otherwise falls back to the system pager.
 
+### Watch a file
+
+```bash
+rp --watch README.md
+rp --watch --type json data.txt
+```
+
+`--watch` is file-only. It does not support stdin or `-`, disables the pager, redraws the screen when the file changes, and keeps retrying if the file becomes missing or temporarily unreadable.
+
 ### Show version
 
 ```bash