Add opt-in debug logging across the CLI pipeline

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.

tests/test_render.py

@@ -35,6 +35,10 @@ class TestRenderOptions:
         assert opts.line_numbers is True
         assert opts.pager is False
 
+    def test_default_debug_is_none(self) -> None:
+        opts = RenderOptions()
+        assert opts.debug is None
+
 
 class TestRenderMarkdown:
     """Markdown rendering."""
@@ -238,6 +242,18 @@ class TestPagerBehavior:
         assert len(capture_console.file.getvalue()) > 0
         mock_pager.assert_not_called()
 
+    def test_logs_auto_pager_decision(
+        self, capture_console, sample_python: str
+    ) -> None:
+        messages: list[str] = []
+        opts = RenderOptions(pager=None, debug=messages.append)
+
+        with patch("sys.stdout.isatty", return_value=False):
+            render(sample_python, "python", capture_console, opts)
+
+        assert "pager: auto-detected disabled because stdout is not a TTY" in messages
+        assert "pager: writing directly to the console" in messages
+
 
 class TestPadding:
     """Rendered output is wrapped in padding."""