fix(npm): install the binary on first run when postinstall was skipped

npm 12 blocks install scripts of unapproved packages under `npx` and
`npm install -g` (its allow-scripts policy). The postinstall download never
ran, so `npx -y idx-mcp` exited with "installed binary not found" and MCP
clients saw only "Connection closed". Hermes Agent hits this by default: it
runs a bare `npx` with its own npm 12.

The launcher now runs the installer itself when the binary is missing,
logging to stderr so the idx-mcp stdout channel stays clean. This also covers
pnpm 10+ and bun, which skip dependency scripts by default.

npm-smoke installs idx-mcp with --ignore-scripts and checks that the first
run still starts cleanly.

Fixes #41

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WB13omP6h7YeyJx7GNtUnF

scripts/idx.js

@@ -3,7 +3,7 @@
 
 const fs = require("node:fs");
 const path = require("node:path");
-const { spawn } = require("node:child_process");
+const { spawn, spawnSync } = require("node:child_process");
 
 const packageManifest = JSON.parse(
   fs.readFileSync(path.join(__dirname, "..", "package.json"), "utf8"),
@@ -13,6 +13,15 @@ const packageName = packageManifest.name;
 const binaryName = process.platform === "win32" ? `${nativeBinary}.exe` : nativeBinary;
 const binaryPath = path.join(__dirname, "..", "bin", binaryName);
 
+// postinstall never ran (ignore-scripts) or its download failed: install now.
+// Its log goes to stderr, since stdout is the MCP channel for idx-mcp.
+if (!fs.existsSync(binaryPath)) {
+  console.error(`${packageName}: binary not installed; running the installer`);
+  spawnSync(process.execPath, [path.join(__dirname, "install.js")], {
+    stdio: ["ignore", 2, 2],
+  });
+}
+
 if (!fs.existsSync(binaryPath)) {
   console.error(`${packageName}: installed binary not found at ${binaryPath}`);
   console.error(`${packageName}: rerun npm install or set IDX_BINARY_URL for a local binary`);