fix: add Antigravity prereq check to all setup scripts (#7)

All three setup scripts (Linux, macOS, Windows) now verify that the
Antigravity app is installed and the LS binary exists before proceeding.
Fails early with a clear error message and suggestions instead of a
cryptic runtime crash.

Closes #5

scripts/setup-linux.sh

@@ -18,7 +18,26 @@ if [ ${#MISSING[@]} -gt 0 ]; then
     exit 1
 fi
 
-# ── 1. System user for UID isolation ──
+# ── 1. Prerequisite check: Antigravity must be installed ──
+LS_BINARY="${ZEROGRAVITY_LS_PATH:-/usr/share/antigravity/resources/app/extensions/antigravity/bin/language_server_linux_x64}"
+echo "→ Checking for Antigravity installation…"
+if [ ! -f "$LS_BINARY" ]; then
+    echo ""
+    echo "✗ Antigravity is not installed (or the LS binary is missing)."
+    echo "  ZeroGravity requires a working Antigravity installation."
+    echo "  The Language Server binary is bundled with the Antigravity app"
+    echo "  and cannot be downloaded separately."
+    echo ""
+    echo "  Expected path:"
+    echo "    $LS_BINARY"
+    echo ""
+    echo "  Install Antigravity first, then re-run this script."
+    echo "  Alternatively, set ZEROGRAVITY_LS_PATH to a custom LS binary location."
+    exit 1
+fi
+echo "  Found: $LS_BINARY"
+
+# ── 2. System user for UID isolation ──
 echo "→ Creating zerogravity-ls system user…"
 if id -u zerogravity-ls &>/dev/null; then
     echo "  Already exists."
@@ -27,7 +46,7 @@ else
     echo "  Created."
 fi
 
-# ── 2. Sudoers rule (run commands as zerogravity-ls without password) ──
+# ── 3. Sudoers rule (run commands as zerogravity-ls without password) ──
 SUDOERS="/etc/sudoers.d/zerogravity"
 echo "→ Installing sudoers rule…"
 if [ -f "$SUDOERS" ]; then
@@ -38,16 +57,16 @@ else
     echo "  Installed: $SUDOERS"
 fi
 
-# ── 3. Data directory permissions ──
+# ── 4. Data directory permissions ──
 echo "→ Setting up /tmp/zerogravity-standalone…"
 sudo mkdir -p /tmp/zerogravity-standalone
 sudo chmod 1777 /tmp/zerogravity-standalone
 
-# ── 4. Config directory ──
+# ── 5. Config directory ──
 echo "→ Setting up ~/.config/zerogravity…"
 mkdir -p "$HOME/.config/zerogravity"
 
-# ── 5. Systemd user service ──
+# ── 6. Systemd user service ──
 echo "→ Installing systemd user service…"
 UNIT_DIR="$HOME/.config/systemd/user"
 mkdir -p "$UNIT_DIR"
@@ -73,7 +92,7 @@ systemctl --user daemon-reload
 echo "  Installed: $UNIT_DIR/zerogravity.service"
 echo "  Enable with: systemctl --user enable zerogravity"
 
-# ── 6. Build ──
+# ── 7. Build ──
 echo "→ Building release binary…"
 cd "$PROJECT_DIR"
 cargo build --release 2>&1 | tail -1