chore(cli): add script to find merge conflict markers in a file

Provides a small helper for upstream merge workflows so agents can be
allowlisted to search for conflict markers without needing general shell
access. Wraps the canonical ripgrep invocation with a POSIX grep fallback.

script/upstream/find-conflict-markers.sh

@@ -0,0 +1,48 @@
+#!/usr/bin/env bash
+# Find git merge conflict markers in a file.
+#
+# Prints the line number and the marker for each of:
+#   <<<<<<< (ours start)
+#   ||||||| (base / diff3 separator)
+#   ======= (separator)
+#   >>>>>>> (theirs end)
+#
+# Usage:
+#   script/upstream/find-conflict-markers.sh <file>
+set -euo pipefail
+
+if [ "$#" -ne 1 ]; then
+  echo "Usage: $0 <file>" >&2
+  exit 2
+fi
+
+file=$1
+
+if [ ! -f "$file" ]; then
+  echo "error: not a file: $file" >&2
+  exit 2
+fi
+
+# Match the four conflict marker line shapes. `=======` must be the whole line;
+# the others may have trailing content (branch name, commit hash, etc.).
+#
+# Prefer ripgrep when available (matches the historical invocation), fall back
+# to POSIX grep so the script works in minimal environments.
+if command -v rg >/dev/null 2>&1; then
+  rg -n '^(<{7}|\|{7}|={7}$|>{7})' "$file" || {
+    status=$?
+    # rg exits 1 when no matches are found; treat that as success (clean file).
+    if [ "$status" -eq 1 ]; then
+      exit 0
+    fi
+    exit "$status"
+  }
+else
+  grep -nE '^(<{7}|\|{7}|={7}$|>{7})' "$file" || {
+    status=$?
+    if [ "$status" -eq 1 ]; then
+      exit 0
+    fi
+    exit "$status"
+  }
+fi