test(install): lock uv installer to a resolved PowerShell host

Source-level guard (install.ps1 only runs on Windows, so there's no Linux CI
runner to execute it): the astral uv install line must be invoked via the call
operator on a resolved host variable, the bare-`powershell` literal that
produced the field-reported "The term 'powershell' is not recognized" must be
gone, and the resolver must be PATH-independent (Get-Process -Id $PID) and
pwsh-aware.

tests/test_install_ps1_uv_powershell_host.py

@@ -0,0 +1,77 @@
+"""Regression: the Windows installer must not spawn a bare ``powershell``.
+
+A user on Windows reported the installer getting stuck; running
+``irm https://hermes-agent.nousresearch.com/install.ps1 | iex`` failed at the
+uv step with::
+
+    [X] Failed to install uv: The term 'powershell' is not recognized as the
+        name of a cmdlet, function, script file, or operable program.
+
+Root cause: ``Install-Uv`` spawned the astral uv installer via a hardcoded
+bare ``powershell`` command.  That name resolves only to *Windows PowerShell*
+and only when its System32 directory is on ``PATH``.  Under PowerShell 7+
+(``pwsh``) -- or any session where ``powershell`` isn't on ``PATH`` -- the
+spawn dies and uv installation aborts.
+
+The fix resolves the PowerShell host executable (preferring the absolute path
+of the running host, then ``powershell``/``pwsh`` via ``Get-Command``) and
+invokes *that* instead of a bare name.  These tests lock that contract at the
+source level (the script only runs on Windows, so there's no runner to
+execute it on Linux CI).
+"""
+
+from pathlib import Path
+
+import pytest
+
+_INSTALL_PS1 = Path(__file__).resolve().parents[1] / "scripts" / "install.ps1"
+
+
+@pytest.fixture(scope="module")
+def source() -> str:
+    return _INSTALL_PS1.read_text(encoding="utf-8")
+
+
+def test_astral_uv_installer_not_spawned_via_bare_powershell(source: str):
+    """The exact failing literal must be gone."""
+    forbidden = 'powershell -ExecutionPolicy ByPass -c "irm https://astral.sh/uv'
+    assert forbidden not in source, (
+        "Install-Uv still spawns the astral uv installer via a bare "
+        "`powershell` — it must use the resolved PowerShell host exe so it "
+        "works under pwsh / when powershell isn't on PATH."
+    )
+
+
+def test_astral_uv_installer_invoked_via_resolved_host_variable(source: str):
+    """The astral uv installer line must use the call operator on a variable.
+
+    i.e. ``& $psHostExe -ExecutionPolicy ... irm https://astral.sh/uv...``
+    rather than naming a fixed executable.
+    """
+    lines = [ln for ln in source.splitlines() if "astral.sh/uv/install.ps1 | iex" in ln]
+    # Exactly one invocation line carries the astral installer.
+    invocation = [ln for ln in lines if "irm https://astral.sh/uv/install.ps1 | iex" in ln]
+    assert invocation, "astral uv install invocation line not found"
+    for ln in invocation:
+        stripped = ln.strip()
+        assert stripped.startswith("& $"), (
+            f"astral uv installer must be invoked via the call operator on a "
+            f"resolved host variable (`& $...`), got: {stripped!r}"
+        )
+
+
+def test_powershell_host_resolver_is_defined_and_portable(source: str):
+    """A host-resolver helper must exist and be PATH-independent + pwsh-aware."""
+    assert "function Get-PowerShellHostExe" in source, (
+        "expected a Get-PowerShellHostExe helper that resolves the host exe"
+    )
+    # PATH-independent: derive the absolute path of the running host.
+    assert "Get-Process -Id $PID" in source, (
+        "resolver must derive the current host's absolute path "
+        "(Get-Process -Id $PID), which is independent of PATH"
+    )
+    # pwsh-aware fallback: PowerShell 7's executable is `pwsh`, not `powershell`.
+    assert "pwsh" in source, (
+        "resolver must fall back to pwsh (PowerShell 7) when powershell is "
+        "unavailable"
+    )