fix: improve error message when PortAudio system library is missing

When sounddevice is installed but libportaudio2 is not present on the
system, the OSError was caught together with ImportError and showed a
generic 'pip install sounddevice' message that sent users down the wrong
path.

Split the except clause to give a clear, actionable message for the
OSError case, including the correct apt/brew commands to install the
system library.

tools/voice_mode.py

@@ -81,8 +81,15 @@ def detect_audio_environment() -> dict:
                 warnings.append("No audio input/output devices detected")
         except Exception:
             warnings.append("Audio subsystem error (PortAudio cannot query devices)")
-    except (ImportError, OSError):
+    except ImportError:
         warnings.append("Audio libraries not installed (pip install sounddevice numpy)")
+    except OSError:
+        warnings.append(
+            "PortAudio system library not found -- install it first:\n"
+            "  Linux:  sudo apt-get install libportaudio2\n"
+            "  macOS:  brew install portaudio\n"
+            "Then retry /voice on."
+        )
 
     return {
         "available": len(warnings) == 0,