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-macos.sh

@@ -16,7 +16,35 @@ mkdir -p "$CONFIG_DIR"
 echo "→ Setting up /tmp/zerogravity-standalone…"
 mkdir -p /tmp/zerogravity-standalone
 
-# ── 3. Launchd plist ──
+# ── 3. Prerequisite check: Antigravity must be installed ──
+LS_BINARY="${ZEROGRAVITY_LS_PATH:-}"
+if [ -z "$LS_BINARY" ]; then
+    # Check both /Applications and ~/Applications
+    for base in "/Applications/Antigravity.app" "$HOME/Applications/Antigravity.app"; do
+        candidate="$base/Contents/Resources/app/extensions/antigravity/bin/language_server_darwin_arm64"
+        if [ -f "$candidate" ]; then
+            LS_BINARY="$candidate"
+            break
+        fi
+    done
+fi
+echo "→ Checking for Antigravity installation…"
+if [ -z "$LS_BINARY" ] || [ ! -f "$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 in: /Applications/Antigravity.app or ~/Applications/Antigravity.app"
+    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: $LS_BINARY"
+
+# ── 4. Launchd plist ──
 echo "→ Installing launchd plist…"
 PLIST_DIR="$HOME/Library/LaunchAgents"
 PLIST="$PLIST_DIR/com.zerogravity.proxy.plist"
@@ -55,7 +83,7 @@ echo "  Installed: $PLIST"
 echo "  Start with: launchctl load $PLIST"
 echo "  Stop with:  launchctl unload $PLIST"
 
-# ── 4. Build ──
+# ── 5. Build ──
 echo "→ Building release binary…"
 cd "$PROJECT_DIR"
 cargo build --release 2>&1 | tail -1