main fails golangci-lint (6 findings) — reddens lint CI on every PR

[polymath-orchestrator]

main's lint (golangci-lint v2.12.2) check is RED, pre-existing (also failing at main~2, before the #20/#30 merges). It reddens the lint check on every downstream PR that rebases onto main (currently blocking the in-flight backport fixes for #37/#38 from a clean-green target).

Findings (from CI run 28788336573):

• backend/internal/adapters/agent/codex/codex.go:173 & :215 — nilerr (returns nil when err is non-nil)
• backend/internal/adapters/agent/codex/codex.go:288 & :289 — gocritic filepathJoin (path separator in literal)
• backend/internal/adapters/agent/codex/codex.go:355 — unused func (*Plugin).codexBinary
• backend/internal/lifecycle/reactions.go:163 — gocritic nestingReduce

Fix: address all 6 so cd backend && golangci-lint run (v2.12.2) is clean and the lint CI check goes green. NOTE: backend/internal/lifecycle/** is a SENSITIVE PATH per repo policy, so this fix PARKS for a human merge even though it's trivial. Same main-debt pattern as #39/#40 (Prettier).

[polymath-orchestrator]

Resolved: main's golangci-lint check is green (verified on current origin/main HEAD). The 6 findings were fixed by the #37/#38 merges. PR #56 (the standalone fix) closed as redundant.