fix(windows): use PortableGit (not MinGit), fix relaunch os.execvp crash, surface npm errors

Three real bugs from teknium1's first Windows install run:

1. **MinGit has no bash.exe.**  MinGit is the minimal-automation Git for Windows
   distribution — it ships git.exe but deliberately strips bash and the POSIX
   coreutils.  Installer logged "Could not locate bash.exe" and Hermes would
   fail to run any shell command.  Switched to PortableGit — the full Git for
   Windows minus the installer UI.  PortableGit ships bash.exe at
   <root>\bin\bash.exe plus sh, awk, sed, grep, curl, ssh in usr\bin\.  ARM64
   variant is detected separately (PortableGit-*-arm64.7z.exe).  32-bit falls
   back to MinGit-32-bit with a warning (PortableGit is 64-bit only).

   PortableGit ships as a 7z self-extractor (56MB vs MinGit's 38MB).  We
   invoke it with `-o<target> -y` to extract silently — no 7z install needed,
   it's self-contained.

   Updated tools/environments/local.py::_find_bash candidate order to prefer
   the PortableGit layout (<root>\bin\bash.exe) with the MinGit layout
   (<root>\usr\bin\bash.exe) as a fallback so existing installs keep working.

2. **os.execvp "Exec format error" on Windows.**  Setup wizard's "Launch
   hermes chat now? Y" called `os.execvp(["hermes", "chat"])` which on
   Windows can only swap to real Win32 .exe files — chokes with OSError(8)
   on .cmd batch shims and Python console-script wrappers.  Added a
   win32 branch in hermes_cli/relaunch.py::relaunch() that uses
   subprocess.run + sys.exit — functionally identical (user sees "hermes
   exited, then new hermes started") with one extra PID in play.  POSIX
   path is UNCHANGED — still uses os.execvp for in-place replacement.
   Catches OSError in the Windows branch and surfaces a "open a new
   terminal so PATH picks up, then re-run hermes" hint instead of a
   cryptic traceback.

3. **npm install failures silent on Windows.**  The install.ps1 was invoking
   `npm install --silent 2>&1 | Out-Null` inside a try/catch.  PowerShell's
   try/catch does NOT trigger on non-zero process exit codes — only on
   unhandled .NET exceptions — so npm failing printed a generic "npm
   install failed" with zero information about WHY.  The silent pipe ate
   the stderr.

   Rewrote Install-NodeDeps to:
   - Resolve npm.cmd via Get-Command (respects PATHEXT) instead of
     relying on bare `npm` name resolution.
   - Use Start-Process with -PassThru to capture the actual exit code.
   - Redirect stderr to a temp log and surface the first ~800 chars of
     the real npm error when install fails, plus the log path for the
     full text.
   - Fail loudly with the right exit code instead of a misleading success.
   - Bail cleanly with a helpful message when npm isn't on PATH at all.

4. **"True" printing to console after Node check.**  `Test-Node` returns $true;
   installer called it as a bare statement (no assignment, no cast).  PowerShell
   prints bare return values.  Wrapped the call in `[void](Test-Node)`.

## Tests

- Added 3 new tests in tests/hermes_cli/test_relaunch.py covering the
  Windows branch: subprocess is called (not execvp), child exit code
  propagates, OSError surfaces a helpful message.  All 23 tests pass
  (20 existing + 3 new).
- 77 Windows-compat tests still pass, POSIX behaviour unchanged.

tests/hermes_cli/test_relaunch.py

@@ -152,4 +152,81 @@ class TestRelaunch:
         with pytest.raises(SystemExit):
             relaunch_mod.relaunch(["--resume", "abc"])
 
-        assert calls == [("/usr/bin/hermes", ["/usr/bin/hermes", "--resume", "abc"])]
+        assert calls == [("/usr/bin/hermes", ["/usr/bin/hermes", "--resume", "abc"])]
+
+    def test_windows_uses_subprocess_not_execvp(self, monkeypatch):
+        """On Windows, os.execvp raises OSError "Exec format error" when the
+        target is a .cmd shim or console-script wrapper (both common for
+        hermes).  relaunch() must detect win32 and use subprocess.run +
+        sys.exit instead."""
+        monkeypatch.setattr(relaunch_mod.sys, "platform", "win32")
+        monkeypatch.setattr(relaunch_mod, "resolve_hermes_bin", lambda: r"C:\Users\test\hermes.exe")
+
+        import subprocess as _subprocess
+
+        captured_argv = []
+
+        def fake_subprocess_run(argv, **kwargs):
+            captured_argv.append(list(argv))
+            class _Result:
+                returncode = 0
+            return _Result()
+
+        monkeypatch.setattr(_subprocess, "run", fake_subprocess_run)
+
+        # execvp MUST NOT be called on Windows — route must go through subprocess
+        execvp_calls = []
+
+        def fake_execvp(*args, **kwargs):
+            execvp_calls.append(args)
+            raise AssertionError("os.execvp must not be called on Windows")
+
+        monkeypatch.setattr(relaunch_mod.os, "execvp", fake_execvp)
+
+        with pytest.raises(SystemExit) as exc_info:
+            relaunch_mod.relaunch(["chat"])
+
+        assert exc_info.value.code == 0
+        assert execvp_calls == []
+        assert captured_argv == [[r"C:\Users\test\hermes.exe", "chat"]]
+
+    def test_windows_propagates_child_exit_code(self, monkeypatch):
+        """A non-zero exit from the child should flow through to sys.exit."""
+        monkeypatch.setattr(relaunch_mod.sys, "platform", "win32")
+        monkeypatch.setattr(relaunch_mod, "resolve_hermes_bin", lambda: r"C:\hermes.exe")
+
+        import subprocess as _subprocess
+
+        def fake_run(argv, **kwargs):
+            class _Result:
+                returncode = 42
+            return _Result()
+
+        monkeypatch.setattr(_subprocess, "run", fake_run)
+        monkeypatch.setattr(relaunch_mod.os, "execvp", lambda *a, **kw: None)
+
+        with pytest.raises(SystemExit) as exc_info:
+            relaunch_mod.relaunch(["chat"])
+        assert exc_info.value.code == 42
+
+    def test_windows_surfaces_oserror_with_help(self, monkeypatch, capsys):
+        """When subprocess itself raises OSError (file-not-found / bad format),
+        we must NOT let it bubble up as a cryptic traceback — print a
+        user-readable hint and sys.exit(1)."""
+        monkeypatch.setattr(relaunch_mod.sys, "platform", "win32")
+        monkeypatch.setattr(relaunch_mod, "resolve_hermes_bin", lambda: r"C:\missing.exe")
+
+        import subprocess as _subprocess
+
+        def fake_run(argv, **kwargs):
+            raise OSError(2, "No such file or directory")
+
+        monkeypatch.setattr(_subprocess, "run", fake_run)
+        monkeypatch.setattr(relaunch_mod.os, "execvp", lambda *a, **kw: None)
+
+        with pytest.raises(SystemExit) as exc_info:
+            relaunch_mod.relaunch(["chat"])
+        assert exc_info.value.code == 1
+        err = capsys.readouterr().err
+        assert "relaunch failed" in err
+        assert "open a new terminal" in err.lower() or "path" in err.lower()