chore: add operational notes and test shell source

apps/test_shell.c

@@ -0,0 +1,33 @@
+// Minimal test shell to verify the execution environment
+#include <stddef.h>
+
+extern int write(int fd, const void *buf, size_t count);
+extern int read(int fd, void *buf, size_t count);
+
+int main(int argc, char *argv[], char *envp[]) {
+    const char *prompt = "shell> ";
+    char buf[128];
+    
+    while (1) {
+        // Print prompt
+        write(1, prompt, 7);
+        
+        // Read command
+        int n = read(0, buf, sizeof(buf) - 1);
+        if (n <= 0) continue;
+        
+        buf[n] = '\0';
+        
+        // Echo back
+        write(1, "You typed: ", 11);
+        write(1, buf, n);
+        
+        // Check for exit
+        if (buf[0] == 'q' && buf[1] == '\n') {
+            write(1, "Goodbye!\n", 9);
+            break;
+        }
+    }
+    
+    return 0;
+}