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

netinfra/gather-configs.sh

@@ -44,7 +44,7 @@ if command -v sqlite3 >/dev/null 2>&1; then
     echo "-- adlist count --"; sqlite3 -readonly "$G" "SELECT COUNT(*) FROM adlist;" 2>&1
     echo "-- domainlist count by type --"; sqlite3 -readonly "$G" "SELECT type,COUNT(*) FROM domainlist GROUP BY type;" 2>&1
     echo "-- domainlist (allow=0/allow_exact, deny=1/deny_exact, etc.) first 80 --"; sqlite3 -readonly "$G" "SELECT type,domain,enabled,comment FROM domainlist LIMIT 80;" 2>&1
-    echo -- client --"; sqlite3 -readonly "$G" "SELECT ip,comment FROM client;" 2>&1
+    echo "-- client --"; sqlite3 -readonly "$G" "SELECT ip,comment FROM client;" 2>&1
     echo "-- group --"; sqlite3 -readonly "$G" "SELECT id,name,enabled,comment FROM 'group';" 2>&1
     echo "-- info --"; sqlite3 -readonly "$G" "SELECT * FROM info;" 2>&1
 else