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

tests/test_cli.py

@@ -5,6 +5,7 @@ from __future__ import annotations
 from pathlib import Path
 from unittest.mock import patch
 
+import pytest
 from typer.testing import CliRunner
 
 from rp import __version__
@@ -171,6 +172,18 @@ class TestStdinInput:
 
         assert result.exit_code == 0
 
+    def test_watch_rejects_stdin(self) -> None:
+        result = runner.invoke(app, ["--watch"], input="hello\n")
+
+        assert result.exit_code == 1
+        assert "--watch only supports file input" in result.output
+
+    def test_watch_rejects_dash_input(self) -> None:
+        result = runner.invoke(app, ["-", "--watch"], input="hello\n")
+
+        assert result.exit_code == 1
+        assert "--watch only supports file input" in result.output
+
     def test_stdin_passes_none_path(self) -> None:
         """When reading from stdin, detect_type should receive path=None."""
         with patch("rp.cli.detect_type") as mock_detect_type:
@@ -311,3 +324,96 @@ class TestLineNumbersOption:
         result = runner.invoke(app, [str(file), "--no-line-numbers", "--no-pager"])
 
         assert result.exit_code == 0
+
+
+class TestWatchMode:
+    """Tests for the watch mode loop."""
+
+    def test_watch_disables_pager(self, tmp_file) -> None:
+        file = tmp_file("x = 1\n", suffix=".py")
+
+        with patch("rp.cli.detect_type", return_value="python"):
+            with patch("rp.cli.time.sleep", side_effect=KeyboardInterrupt):
+                with patch("rp.cli.render") as mock_render:
+                    with pytest.raises(KeyboardInterrupt):
+                        main(file=file, pager=True, watch=True)
+
+        call_args = mock_render.call_args
+        assert call_args is not None
+        options = call_args[0][3]
+        assert options.pager is False
+
+    def test_watch_rerenders_on_file_change(self, tmp_file) -> None:
+        file = tmp_file("x = 1\n", suffix=".py")
+        sleep_calls = 0
+
+        def fake_sleep(_: float) -> None:
+            nonlocal sleep_calls
+            sleep_calls += 1
+            if sleep_calls == 1:
+                file.write_text("x = 2\n", encoding="utf-8")
+                return
+            raise KeyboardInterrupt
+
+        with patch("rp.cli.detect_type", return_value="python"):
+            with patch("rp.cli.time.sleep", side_effect=fake_sleep):
+                with patch("rp.cli.render") as mock_render:
+                    with pytest.raises(KeyboardInterrupt):
+                        main(file=file, watch=True)
+
+        assert [call.args[0] for call in mock_render.call_args_list] == [
+            "x = 1\n",
+            "x = 2\n",
+        ]
+
+    def test_watch_retries_after_missing_file_and_recovers(self, tmp_file) -> None:
+        file = tmp_file("x = 1\n", suffix=".py")
+        sleep_calls = 0
+
+        def fake_sleep(_: float) -> None:
+            nonlocal sleep_calls
+            sleep_calls += 1
+            if sleep_calls == 1:
+                file.unlink()
+                return
+            if sleep_calls == 2:
+                file.write_text("x = 3\n", encoding="utf-8")
+                return
+            raise KeyboardInterrupt
+
+        with patch("rp.cli.detect_type", return_value="python"):
+            with patch("rp.cli.time.sleep", side_effect=fake_sleep):
+                with patch("rp.cli.Console.print") as mock_print:
+                    with patch("rp.cli.render") as mock_render:
+                        with pytest.raises(KeyboardInterrupt):
+                            main(file=file, watch=True)
+
+        assert [call.args[0] for call in mock_render.call_args_list] == [
+            "x = 1\n",
+            "x = 3\n",
+        ]
+        mock_print.assert_any_call(f"[red]Error:[/red] File not found: {file}")
+
+    def test_watch_recovers_when_file_is_missing_at_startup(
+        self, tmp_path: Path
+    ) -> None:
+        file = tmp_path / "startup.py"
+        sleep_calls = 0
+
+        def fake_sleep(_: float) -> None:
+            nonlocal sleep_calls
+            sleep_calls += 1
+            if sleep_calls == 1:
+                file.write_text("x = 4\n", encoding="utf-8")
+                return
+            raise KeyboardInterrupt
+
+        with patch("rp.cli.detect_type", return_value="python"):
+            with patch("rp.cli.time.sleep", side_effect=fake_sleep):
+                with patch("rp.cli.Console.print") as mock_print:
+                    with patch("rp.cli.render") as mock_render:
+                        with pytest.raises(KeyboardInterrupt):
+                            main(file=file, watch=True)
+
+        assert [call.args[0] for call in mock_render.call_args_list] == ["x = 4\n"]
+        mock_print.assert_any_call(f"[red]Error:[/red] File not found: {file}")