fix(windows): use shutil.which() for npm subprocess calls (#73)

Two subprocess.run(["npm", ...]) calls in undici installation were
using bare command name, which fails on Windows where npm is
installed as npm.cmd. Now resolves full path via shutil.which()
first, consistent with all other subprocess calls.

Co-authored-by: Panniantong <panniantong@users.noreply.github.com>

agent_reach/cli.py

@@ -410,14 +410,15 @@ def _install_system_deps():
             print("  ⬜ xreach CLI requires Node.js (optional — Twitter reading still works via Jina)")
 
     # ── undici (proxy support for Node.js fetch) ──
-    if shutil.which("npm"):
-        npm_root = subprocess.run(["npm", "root", "-g"], capture_output=True, encoding="utf-8", errors="replace", timeout=5).stdout.strip()
+    npm_cmd = shutil.which("npm")
+    if npm_cmd:
+        npm_root = subprocess.run([npm_cmd, "root", "-g"], capture_output=True, encoding="utf-8", errors="replace", timeout=5).stdout.strip()
         undici_path = os.path.join(npm_root, "undici", "index.js") if npm_root else ""
         if os.path.exists(undici_path):
             print("  ✅ undici already installed (Node.js proxy support)")
         else:
             try:
-                subprocess.run(["npm", "install", "-g", "undici"], capture_output=True, encoding="utf-8", errors="replace", timeout=60)
+                subprocess.run([npm_cmd, "install", "-g", "undici"], capture_output=True, encoding="utf-8", errors="replace", timeout=60)
                 print("  ✅ undici installed (Node.js proxy support)")
             except Exception:
                 print("  ⬜ undici install failed (optional — xreach may not work behind proxies)")