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_debug.py

@@ -0,0 +1,60 @@
+"""Tests for debug helpers."""
+
+from __future__ import annotations
+
+import io
+
+from rich.console import Console
+
+from rp.debug import DEBUG_ENV_VAR, env_debug_enabled, log_debug, make_debug_logger
+
+
+class TestEnvDebugEnabled:
+    """Environment-based debug mode toggles."""
+
+    def test_disabled_by_default(self) -> None:
+        assert env_debug_enabled({}) is False
+
+    def test_false_like_values_disable_debug(self) -> None:
+        for value in ["", "0", "false", "no", "off"]:
+            assert env_debug_enabled({DEBUG_ENV_VAR: value}) is False
+
+    def test_false_like_values_are_trimmed_and_case_insensitive(self) -> None:
+        for value in [" False ", "\tNO\n", " Off "]:
+            assert env_debug_enabled({DEBUG_ENV_VAR: value}) is False
+
+    def test_truthy_values_enable_debug(self) -> None:
+        assert env_debug_enabled({DEBUG_ENV_VAR: "1"}) is True
+        assert env_debug_enabled({DEBUG_ENV_VAR: "true"}) is True
+
+    def test_truthy_values_are_trimmed_and_case_insensitive(self) -> None:
+        for value in [" TRUE ", " Yes ", "\ton\n"]:
+            assert env_debug_enabled({DEBUG_ENV_VAR: value}) is True
+
+
+class TestMakeDebugLogger:
+    """Console-backed debug logger output."""
+
+    def test_logs_visible_prefix(self) -> None:
+        stream = io.StringIO()
+        console = Console(file=stream, force_terminal=False)
+        logger = make_debug_logger(console, enabled=True)
+
+        assert logger is not None
+        logger("hello")
+
+        assert stream.getvalue() == "[rp debug] hello\n"
+
+
+class TestLogDebug:
+    """Nil-safe debug logger forwarding."""
+
+    def test_noop_when_logger_is_none(self) -> None:
+        log_debug(None, "hello")
+
+    def test_forwards_to_logger(self) -> None:
+        messages: list[str] = []
+
+        log_debug(messages.append, "hello")
+
+        assert messages == ["hello"]