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

provisioning/Dell/Server/fixeth.sh

@@ -5,17 +5,18 @@
 #magic to detect main int
 echo "Determining management interface..."
 #export MAIN_INT=$(brctl show $(netstat -rn|grep 0.0.0.0|head -n1|awk '{print $NF}') | awk '{print $NF}'|tail -1|awk -F '.' '{print $1}')
-export MAIN_INT=$(brctl show|grep vmbr0|awk '{print $NF}'|awk -F '.' '{print $1}')
+MAIN_INT=$(brctl show|grep vmbr0|awk '{print $NF}'|awk -F '.' '{print $1}')
+export MAIN_INT
 
 echo "Management interface is: $MAIN_INT"
 
 #fix the issue
 echo "Fixing management interface..."
-ethtool -K $MAIN_INT tso off 
-ethtool -K $MAIN_INT gro off 
-ethtool -K $MAIN_INT gso off
-ethtool -K $MAIN_INT tx off
-ethtool -K $MAIN_INT rx off
+ethtool -K "$MAIN_INT" tso off 
+ethtool -K "$MAIN_INT" gro off 
+ethtool -K "$MAIN_INT" gso off
+ethtool -K "$MAIN_INT" tx off
+ethtool -K "$MAIN_INT" rx off
 
 #https://forum.proxmox.com/threads/e1000-driver-hang.58284/
 #https://serverfault.com/questions/616485/e1000e-reset-adapter-unexpectedly-detected-hardware-unit-hang