Add lintrunner pre-commit hook

Summary:
Run lintrunner automatically on every commit via .githooks/pre-commit.
Hooks are enabled by default through install_executorch.sh.

Test Plan:
Verified lint violations block commits and clean files pass.

Author: Abhinay Kukkadapu

.githooks/pre-commit

@@ -27,4 +27,25 @@ if git diff --cached --name-only | grep -q "^torch_pin.py$"; then
     fi
 fi
 
-exit 0
+# --- lintrunner ---
+
+if ! command -v lintrunner >/dev/null 2>&1; then
+    echo "Warning: lintrunner not found. Skipping lint checks."
+    echo "Install with: pip install lintrunner lintrunner-adapters && lintrunner init"
+    exit 0
+fi
+
+staged_files=$(git diff --cached --name-only --diff-filter=ACMR)
+
+lintrunner -a --revision 'HEAD^' --skip MYPY
+lint_status=$?
+
+# Re-stage only paths that were already staged before lintrunner ran.
+while IFS= read -r path; do
+    [ -z "${path}" ] && continue
+    if [ -f "${path}" ]; then
+        git add -- "${path}" || true
+    fi
+done <<< "${staged_files}"
+
+exit $lint_status

install_executorch.sh

@@ -10,3 +10,8 @@
 # so we avoid repeated symlink segments in downstream CMake paths.
 cd -- "$( realpath "$( dirname -- "${BASH_SOURCE[0]}" )" )" &> /dev/null || /bin/true
 ./run_python_script.sh ./install_executorch.py "$@"
+
+# Install git hooks if inside a git repo
+if git rev-parse --git-dir > /dev/null 2>&1; then
+    git config core.hooksPath .githooks
+fi