v1.8.71: fix Windows compatibility — guard Linux-only X11 code with platform check

- main.py: wrap _ensure_display_access() in sys.platform != "win32" check
- main.py: add FileNotFoundError to except in _find_active_xauthority()
- claude_setup.py: platform-aware error message for ssh-keygen not found
- CLAUDE.md: add mandatory cross-platform rules for all contributors

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

core/claude_setup.py

@@ -123,7 +123,8 @@ def generate_ssh_key() -> str:
         log.info(f"SSH key generated: {SSH_KEY_PATH}")
         return f"Key generated: {SSH_KEY_PATH}"
     except FileNotFoundError:
-        msg = "ERROR: ssh-keygen not found — install openssh-client"
+        hint = "enable OpenSSH optional feature" if sys.platform == "win32" else "install openssh-client"
+        msg = f"ERROR: ssh-keygen not found — {hint}"
         log.error(msg)
         return msg
     except subprocess.CalledProcessError as e: