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.
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