feat: ensure-agent + resolve npm-global CLIs without PATH

Add guided multi-select ensure-agent, resolve backend binaries via npm
prefix when not on PATH, and optionally persist npm bin in shell rc.

tests/test_ensure_backend.py

@@ -2,6 +2,7 @@
 
 from __future__ import annotations
 
+import os
 from pathlib import Path
 from unittest.mock import patch
 
@@ -63,21 +64,31 @@ class TestEnsureBackend:
             assert launcher_mod.ensure_backend("hermes") is False
 
 
-class TestResolveInstalledBinary:
-    def test_falls_back_to_npm_global_bin(self, tmp_path: Path) -> None:
+class TestResolveBackendBinary:
+    def test_falls_back_to_npm_global_bin(self, tmp_path: Path, monkeypatch) -> None:
         bin_dir = tmp_path / "bin"
         bin_dir.mkdir()
         claude = bin_dir / "claude"
         claude.write_text("#!/bin/sh\n")
         claude.chmod(0o755)
-        cfg = launcher_mod._BACKENDS["claude"]
+        monkeypatch.delenv("CLAUDE_BIN", raising=False)
         with (
-            patch.object(
-                launcher_mod, "_resolve_backend_binary", return_value=None
-            ),
+            patch.object(launcher_mod.shutil, "which", return_value=None),
             patch.object(launcher_mod, "_npm_global_bin", return_value=bin_dir),
         ):
-            assert launcher_mod._resolve_installed_binary(cfg) == str(claude)
+            assert launcher_mod.resolve_backend_binary("claude") == str(claude)
+
+    def test_ensure_npm_bin_prepends_path(
+        self, tmp_path: Path, monkeypatch
+    ) -> None:
+        bin_dir = tmp_path / "npm-bin"
+        bin_dir.mkdir()
+        monkeypatch.setenv("PATH", "/usr/bin")
+        with patch.object(launcher_mod, "_npm_global_bin", return_value=bin_dir):
+            got = launcher_mod.ensure_npm_bin_on_path(persist=False)
+        assert got == bin_dir
+        assert str(bin_dir) in os.environ["PATH"].split(os.pathsep)
+        assert os.environ["PATH"].split(os.pathsep)[0] == str(bin_dir)
 
 
 class TestOfferInstallGuided: