fix: address review findings (rollback mainline, audit flag, confidence gate, test pattern, jq args)

- git revert uses --mainline 1 for merge commits
- inject dispatch passes all args so --audit flag is reachable
- confidence gate defaults to 0.0 (triggers gate) instead of 0.7 (bypasses)
- test-first grep uses word-boundary patterns to avoid false positives
- jq uses --arg instead of string interpolation for safe filtering

lib/archeflow-rollback.sh

@@ -48,7 +48,7 @@ if timeout 300 bash -c "$TEST_CMD"; then
 fi
 
 echo "Tests FAILED — reverting merge..."
-git revert --no-edit HEAD
+git revert --no-edit --mainline 1 HEAD
 
 # Emit event if event script exists
 if [[ -x "$SCRIPT_DIR/archeflow-event.sh" ]]; then