autoresearch iter 31: isolate lint result cache per checkout in harness

.auto/checks.sh

@@ -4,6 +4,13 @@
 set -uo pipefail
 ROOT="$(cd "$(dirname "$0")/.." && pwd)"
 cd "${ROOT}/backend"
+
+# Same per-checkout lint cache as measure.sh: golangci-lint's default cache is
+# machine-wide, so a different worktree analysing identical sources can make this
+# Guard read back a stale verdict. Key it to the backend directory.
+GOLANGCI_LINT_CACHE="${TMPDIR:-/tmp}/ar-lint-cache-$(pwd | cksum | awk '{print $1}')"
+export GOLANGCI_LINT_CACHE
+
 STATUS=0
 fail() { echo "GUARD FAIL: $1"; STATUS=1; }
 

.auto/measure.sh

@@ -9,6 +9,14 @@
 set -uo pipefail
 cd "$(dirname "$0")/../backend"
 
+# Hermetic lint result cache. golangci-lint's default cache is machine-wide, so
+# entries written while analysing a different worktree are replayed carrying that
+# checkout's absolute paths, which misattributes findings and can serve a stale
+# verdict. Key the cache to this directory. Count-neutral: cold and shared-warm
+# runs both report the same number of findings.
+GOLANGCI_LINT_CACHE="${TMPDIR:-/tmp}/ar-lint-cache-$(pwd | cksum | awk '{print $1}')"
+export GOLANGCI_LINT_CACHE
+
 REF_CFG="$(cd .. && pwd)/.auto/lint.ref.yaml"
 
 # Primary: pinned yardstick findings (never the mutable project config).