harness(fault-proxy): repo-relative RESULT_FILE/CX_SUMMARY paths (drop hardcoded absolute paths)

Env-overridable defaults under $ART (the gitignored artifacts dir) so the
harness is portable and carries no machine-specific absolute paths.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: dzhelezov

harness/chaos/fault-proxy-campaign.sh

@@ -35,8 +35,8 @@ BASELINE_DBNAME="$CHAOS_BASELINE_DBNAME"
 BASELINE_URL="postgres://postgres@$PGHOST_TCP:$PGPORT/$BASELINE_DBNAME"
 BASELINE_META="$CHAOS_BASELINE_META"
 AGG="$ART/aggregate.json"
-RESULT_FILE="/home/claude-ops/ops/p1-fault-proxy/RESULT.md"
-CX_SUMMARY="/tmp/cx-p1.md"
+RESULT_FILE="${CHAOS_RESULT_FILE:-$ART/RESULT.md}"
+CX_SUMMARY="${CHAOS_CX_SUMMARY:-$ART/cx-summary.md}"
 BACKEND_LABEL=""
 PROXY_PID=""
 BASELINE_WALL_SEC=600