fix(api): improve error handling and quoting in agent scripts

- Updated `evofw-firewall.sh` to correctly handle exit codes from the `curl_policy` function, ensuring proper script termination based on policy retrieval status.
- Enhanced `install.sh` to always use single quotes for configuration values, improving safety for names with spaces, and refined the logic for updating the `KERNEL_BACKEND` in the configuration file.
- Updated documentation to reflect the changes in quoting and exit behavior during installation and synchronization processes.

apps/api/src/agent-scripts/evofw-firewall.sh

@@ -43,9 +43,13 @@ curl_policy() {
   return 0
 }
 
-if ! curl_policy "$POLICY_FILE"; then
-  rc=$?
-  [[ "$rc" == "2" ]] && exit 0
+# Do not use `if ! cmd; rc=$?` — after `!`, $? is 0, not the real status.
+policy_rc=0
+curl_policy "$POLICY_FILE" || policy_rc=$?
+if [[ "$policy_rc" -eq 2 ]]; then
+  exit 0
+fi
+if [[ "$policy_rc" -ne 0 ]]; then
   exit 1
 fi