feat:system: Add system utility library with executable check

Introduce a new `system-lib.el` module providing low-level system
utility functions, including function `cj/executable-exists-p` to
check for the availability of programs in PATH. Integrate this
library in `init.el`.

test(system): Add unit tests for executable check function

Create comprehensive unit tests for `cj/executable-exists-p` in
`system-lib.el`, ensuring coverage of normal, boundary and error
scenarios.

1 files changed, 20 insertions, 0 deletions

diff --git a/modules/system-lib.el b/modules/system-lib.el
new file mode 100644
index 00000000..8bbf8474
--- /dev/null
+++ b/modules/system-lib.el
@@ -0,0 +1,20 @@
+;;; system-lib.el --- System utility library functions -*- lexical-binding: t; -*-
+;;
+;;; Commentary:
+;; This module provides low-level system utility functions for checking
+;; the availability of external programs and system capabilities.
+;;
+;; Functions include:
+;; - Checking if external programs are available in PATH
+;;
+;;; Code:
+
+(defun cj/executable-exists-p (program)
+  "Return non-nil if PROGRAM is available in PATH.
+PROGRAM should be a string naming an executable program."
+  (and (stringp program)
+       (not (string-empty-p program))
+       (executable-find program)))
+
+(provide 'system-lib)
+;;; system-lib.el ends here