chore: add macOS universal-ctags v6.1.0 install script and update contributing docs (#1023)

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: brendan-kellam

CONTRIBUTING.md

@@ -36,15 +36,25 @@ Want to take on an issue? Leave a comment and a maintainer may assign it to you
 
 1. Install <a href="https://go.dev/doc/install"><img src="https://go.dev/favicon.ico" width="16" height="16"> go</a>, <a href="https://docs.docker.com/get-started/get-docker/"><img src="https://www.docker.com/favicon.ico" width="16" height="16"> docker</a>, and <a href="https://nodejs.org/"><img src="https://nodejs.org/favicon.ico" width="16" height="16"> NodeJS</a>. Note that a NodeJS version of at least `24` is required.
 
-2. Install [ctags](https://github.com/universal-ctags/ctags) (required by zoekt)
+2. Install [universal-ctags](https://github.com/universal-ctags/ctags) v6.1.0 (required by zoekt). Version 6.1.0 is required — newer versions have a known incompatibility with zoekt.
+
+    **macOS:** Run the provided install script:
     ```sh
-    // macOS:
-    brew install universal-ctags
+    ./install-ctags-macos.sh
+    ```
 
-    // Linux:
-    snap install universal-ctags
+    **Linux and other platforms:** Build from source:
+    ```sh
+    curl https://codeload.github.com/universal-ctags/ctags/tar.gz/v6.1.0 | tar xz -C /tmp
+    cd /tmp/ctags-6.1.0
+    ./autogen.sh
+    ./configure --program-prefix=universal- --enable-json
+    make -j$(nproc)
+    sudo make install
     ```
 
+    After installing, set `CTAGS_COMMAND` in your `.env.development.local` to the path of the installed binary (e.g. `/usr/local/bin/universal-ctags`).
+
 3. Install corepack:
     ```sh
     npm install -g corepack

install-ctags-macos.sh

@@ -0,0 +1,38 @@
+#!/bin/sh
+
+# This script installs universal-ctags v6.1.0 on macOS.
+# This version is pinned to match the version used in the Sourcebot Docker image.
+# See vendor/zoekt/install-ctags-alpine.sh for the Alpine equivalent.
+
+CTAGS_VERSION=v6.1.0
+CTAGS_ARCHIVE_TOP_LEVEL_DIR=ctags-6.1.0
+INSTALL_DIR=/usr/local/bin
+
+cleanup() {
+  cd /
+  rm -rf /tmp/ctags-$CTAGS_VERSION
+}
+
+trap cleanup EXIT
+
+set -eux
+
+# Ensure required build tools are available
+if ! command -v brew >/dev/null 2>&1; then
+  echo "Homebrew is required to install build dependencies. See https://brew.sh."
+  exit 1
+fi
+
+brew install autoconf automake pkg-config jansson
+
+curl --retry 5 "https://codeload.github.com/universal-ctags/ctags/tar.gz/$CTAGS_VERSION" | tar xz -C /tmp
+cd /tmp/$CTAGS_ARCHIVE_TOP_LEVEL_DIR
+./autogen.sh
+./configure --program-prefix=universal- --enable-json
+make -j"$(sysctl -n hw.ncpu)"
+sudo make install
+
+echo ""
+echo "universal-ctags installed to $INSTALL_DIR/universal-ctags"
+echo "Add the following to your .env.development.local:"
+echo "  CTAGS_COMMAND=$INSTALL_DIR/universal-ctags"