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-windows.ps1

@@ -19,7 +19,26 @@ $DataDir = Join-Path $env:TEMP "zerogravity-standalone"
 New-Item -ItemType Directory -Force -Path $DataDir | Out-Null
 Write-Host "  $DataDir"
 
-# ── 3. Build ──
+# ── 3. Prerequisite check: Antigravity must be installed ──
+Write-Host "→ Checking for Antigravity installation…"
+$LsBinary = Join-Path $env:LOCALAPPDATA "Programs\Antigravity\resources\app\extensions\antigravity\bin\language_server_windows_x64.exe"
+if (-not (Test-Path $LsBinary)) {
+    Write-Host ""
+    Write-Host "✗ Antigravity is not installed (or the LS binary is missing)." -ForegroundColor Red
+    Write-Host "  ZeroGravity requires a working Antigravity installation."
+    Write-Host "  The Language Server binary is bundled with the Antigravity app"
+    Write-Host "  and cannot be downloaded separately."
+    Write-Host ""
+    Write-Host "  Expected path:"
+    Write-Host "    $LsBinary"
+    Write-Host ""
+    Write-Host "  Install Antigravity first, then re-run this script."
+    Write-Host "  Alternatively, set ZEROGRAVITY_LS_PATH to a custom LS binary location."
+    exit 1
+}
+Write-Host "  Found: $LsBinary"
+
+# ── 4. Build ──
 Write-Host "→ Building release binary…"
 Push-Location $ProjectDir
 cargo build --release
@@ -32,7 +51,7 @@ if (-not (Test-Path $Binary)) {
 }
 Write-Host "  Built: $Binary"
 
-# ── 4. Scheduled task (optional) ──
+# ── 5. Scheduled task (optional) ──
 $TaskName = "ZeroGravity Proxy"
 $Existing = Get-ScheduledTask -TaskName $TaskName -ErrorAction SilentlyContinue