chore: enforce shellcheck across the repo

Establish shellcheck as a mandatory pre-commit quality gate and bring all 93
shell scripts to a clean state.

- tests/shellcheck.sh: wrapper that runs koalaman/shellcheck:stable via Docker
  (no native binary needed), skips vendored + upstream librenms-agent scripts.
- .shellcheckrc: documents intentional codebase-wide disables (dynamic source
  paths SC1090/SC1091, client-side ssh expansion SC2029).
- AGENTS.md: new Git Policy rule mandating clean shellcheck for every shell
  script before commit.

Fixes applied (real bugs + quality): missing quote in netinfra/gather-configs.sh
(caused cascading parse errors), unquoted expansions, declare-and-assign masking,
egrep -> grep -E, $FUNCNAME array indexing, unused variable removal, cd || exit.
Intentional patterns (sourced config, sysfs/ps diagnostics, ssh heredocs that
expand local config) get justified targeted disables.

💘 Generated with Crush

Assisted-by: Crush:glm-5.2

powerman/setup.sh

@@ -82,7 +82,7 @@ udevadm trigger --subsystem-match=tty 2>/dev/null || true
 sleep 1
 
 if [ -e "/dev/${PDU_DEV_NAME}" ]; then
-  echo "  Device symlink active: /dev/${PDU_DEV_NAME} -> $(readlink -f /dev/${PDU_DEV_NAME})"
+  echo "  Device symlink active: /dev/${PDU_DEV_NAME} -> $(readlink -f "/dev/${PDU_DEV_NAME}")"
 else
   echo "  WARNING: /dev/${PDU_DEV_NAME} not found yet. Adapter may be unplugged."
   echo "  Falling back to /dev/ttyUSB* discovery..."